hmm,
this is a little strange.
I did the very same thing and it worked. The sample app is just a 7.1.5 app: GitHub - mariodavid/cuba71sample
running app looks like this: https://kfz-werkstatt-test.herokuapp.com/#main

I just followed all steps from here: Deployment from GitHub to Heroku - CUBA Platform. Developer’s Manual
Corresponding build logs:
-----> Gradle app detected
-----> Installing JDK 1.8.0_121... done
-----> Building Gradle app...
-----> executing ./gradlew stage
Downloading https://services.gradle.org/distributions/gradle-4.10.3-bin.zip
..........................................................................
> Task :setupTomcat
> Task :app-core:assembleDbScripts
> Task :app-core:beansXml
> Task :app-global:beansXml
> Task :app-global:processResources
> Task :app-global:buildInfo
> Task :app-global:compileJava NO-SOURCE
> Task :app-global:classes
> Task :app-global:jar
> Task :app-core:compileJava NO-SOURCE
> Task :app-core:processResources
> Task :app-core:classes
> Task :app-core:jar
> Task :app-core:sourceJar
> Task :app-core:assemble
> Task :app-core:cleanConf
> Task :app-core:deploy
> Task :app-web:beansXml
> Task :app-web:compileJava NO-SOURCE
> Task :app-web:processResources
> Task :app-web:classes
> Task :app-web:jar
> Task :app-web:sourceJar
> Task :app-web:webArchive
> Task :app-web:assemble
> Task :app-web:cleanConf
> Task :app-web:deploy
> Task :stage
BUILD SUCCESSFUL in 1m 22s
20 actionable tasks: 20 executed
-----> Discovering process types
Procfile declares types -> web
-----> Compressing...
Done: 119.5M
-----> Launching...
Released v5
https://kfz-werkstatt-test.herokuapp.com/ deployed to Heroku
heroku log cli gave me this one:
[cuba71sample] heroku logs --tail --app kfz-werkstatt-test master
› Warning: heroku update available from 7.21.0 to 7.39.1
2020-03-22T17:50:20.155315+00:00 app[api]: Release v1 created by user heroku@die-davids.de
2020-03-22T17:50:20.584900+00:00 app[api]: Release v2 created by user heroku@die-davids.de
2020-03-22T17:50:20.584900+00:00 app[api]: Enable Logplex by user heroku@die-davids.de
2020-03-22T17:50:20.155315+00:00 app[api]: Initial release by user heroku@die-davids.de
2020-03-22T17:55:09.374934+00:00 app[api]: Running release v3 commands by user heroku@die-davids.de
2020-03-22T17:55:09.374934+00:00 app[api]: Attach DATABASE (@ref:postgresql-deep-05618) by user heroku@die-davids.de
2020-03-22T17:55:09.387756+00:00 app[api]: @ref:postgresql-deep-05618 completed provisioning, setting DATABASE_URL. by user heroku@die-davids.de
2020-03-22T17:55:09.387756+00:00 app[api]: Release v4 created by user heroku@die-davids.de
2020-03-22T18:17:04.000000+00:00 app[api]: Build started by user heroku@die-davids.de
2020-03-22T18:18:41.411162+00:00 app[api]: Release v5 created by user heroku@die-davids.de
2020-03-22T18:18:41.411162+00:00 app[api]: Deploy 40491a29 by user heroku@die-davids.de
2020-03-22T18:18:41.434190+00:00 app[api]: Scaled to web@1:Free by user heroku@die-davids.de
2020-03-22T18:18:47.787258+00:00 heroku[web.1]: Starting process with command `cd ./deploy/tomcat/bin && export 'JAVA_OPTS=-Dport.http=58228' && ./catalina.sh run`
2020-03-22T18:18:49.921538+00:00 app[web.1]: Setting JAVA_TOOL_OPTIONS defaults based on dyno size. Custom settings will override them.
2020-03-22T18:18:49.931770+00:00 app[web.1]: Picked up JAVA_TOOL_OPTIONS: -Xmx300m -Xss512k -XX:CICompilerCount=2 -Dfile.encoding=UTF-8
2020-03-22T18:18:50.804163+00:00 app[web.1]: Mar 22, 2020 6:18:50 PM org.apache.catalina.core.AprLifecycleListener lifecycleEvent
2020-03-22T18:18:50.804200+00:00 app[web.1]: INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/app/.jdk/jre/lib/amd64/server::/usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib]
2020-03-22T18:18:50.967723+00:00 app[web.1]: Mar 22, 2020 6:18:50 PM org.apache.coyote.AbstractProtocol init
2020-03-22T18:18:50.967724+00:00 app[web.1]: INFO: Initializing ProtocolHandler ["http-nio-58228"]
2020-03-22T18:18:50.999182+00:00 app[web.1]: Mar 22, 2020 6:18:50 PM org.apache.coyote.AbstractProtocol init
2020-03-22T18:18:50.999183+00:00 app[web.1]: INFO: Initializing ProtocolHandler ["ajp-nio-8009"]
2020-03-22T18:18:51.001780+00:00 app[web.1]: Mar 22, 2020 6:18:51 PM org.apache.catalina.startup.Catalina load
2020-03-22T18:18:51.001782+00:00 app[web.1]: INFO: Server initialization in [565] milliseconds
2020-03-22T18:18:51.455501+00:00 heroku[web.1]: State changed from starting to up
2020-03-22T18:18:51.056081+00:00 app[web.1]: Mar 22, 2020 6:18:51 PM org.apache.catalina.core.StandardService startInternal
2020-03-22T18:18:51.056098+00:00 app[web.1]: INFO: Starting service [Catalina]
2020-03-22T18:18:51.056891+00:00 app[web.1]: Mar 22, 2020 6:18:51 PM org.apache.catalina.core.StandardEngine startInternal
2020-03-22T18:18:51.056894+00:00 app[web.1]: INFO: Starting Servlet engine: [Apache Tomcat/9.0.19]
2020-03-22T18:18:51.065021+00:00 app[web.1]: Mar 22, 2020 6:18:51 PM org.apache.catalina.startup.HostConfig deployDirectory
2020-03-22T18:18:51.065022+00:00 app[web.1]: INFO: Deploying web application directory [/app/deploy/tomcat/webapps/ROOT]
2020-03-22T18:18:51.947516+00:00 app[web.1]: 18:18:51.943 INFO c.h.cuba.core.sys.AppComponents - Using app components: [com.haulmont.cuba]
2020-03-22T18:18:51.988975+00:00 app[web.1]: 18:18:51.988 INFO c.h.c.c.s.AbstractWebAppContextLoader - Loading app properties from classpath:com/company/cuba71sample/web-app.properties
2020-03-22T18:18:52.234424+00:00 app[web.1]: 18:18:52.234 INFO c.h.c.c.s.e.EnvironmentChecksRunner - Environment checks on web module completed successfully
2020-03-22T18:18:54.379207+00:00 app[web.1]: 18:18:54.379 INFO c.h.c.c.s.r.d.StaticServerSelector - Server URLs: [http://localhost:8080/app-core/remoting]
2020-03-22T18:18:54.394480+00:00 app[web.1]: 18:18:54.394 INFO c.h.c.w.s.r.WebRemoteProxyBeanCreator - Configuring proxy beans for local service invocations: [cuba_ConstraintScriptValidationService, cuba_AuthenticationService, cuba_TrustedClientService, cuba_ServerInfoService, cuba_UserSettingService, cuba_UserManagementService, cuba_DataService, cuba_ConfigStorageService, cuba_PersistenceManagerService, cuba_LockService, cuba_UserSessionService, cuba_FoldersService, cuba_EmailService, cuba_FileStorageService, cuba_EntitySnapshotService, cuba_UniqueNumbersService, cuba_LocalizedMessageService, cuba_ResourceService, cuba_ScheduledTasksService, cuba_EntityLogService, cuba_TestingService, cuba_DomainDescriptionService, cuba_RelatedEntitiesService, cuba_EntitySqlGenerationService, cuba_NumberIdService, cuba_QueryResultsService, cuba_DynamicAttributesCacheService, cuba_EntityRestoreService, cuba_LocalFileExchangeService, cuba_EntityImportExportService, cuba_ConstraintLocalizationService, cuba_AttributeAccessService, cuba_ExceptionReportService, cuba_BulkEditorDataService, cuba_AttributeOptionsLoaderService]
2020-03-22T18:18:54.000000+00:00 app[api]: Build succeeded
2020-03-22T18:18:55.826882+00:00 app[web.1]: 18:18:55.826 INFO c.h.c.c.s.CubaThreadPoolTaskScheduler - Initializing ExecutorService 'scheduler'
2020-03-22T18:18:56.093219+00:00 app[web.1]: 18:18:56.093 INFO c.h.cuba.core.sys.jmx.MBeanExporter - Registering beans for JMX exposure: [app.cuba:type=ConfigStorage, app.cuba:type=CachingFacade, app.cuba:type=FileUploading, app.cuba:type=JmxNodeIdentifier, app.cuba:type=JmxLogControl, app.cuba:type=ClassLoaderManager, app.cuba:type=StatisticsCounter]
2020-03-22T18:18:56.223640+00:00 app[web.1]: 18:18:56.223 INFO c.h.cuba.core.sys.MetadataImpl - Initializing metadata
2020-03-22T18:18:56.521959+00:00 app[web.1]: 18:18:56.521 INFO c.h.cuba.core.sys.MetadataImpl - Metadata initialized in 298 ms
2020-03-22T18:18:56.550387+00:00 app[web.1]: 18:18:56.550 INFO c.h.c.c.s.AbstractWebAppContextLoader - AppContext started
2020-03-22T18:18:56.574472+00:00 app[web.1]: 18:18:56.574 INFO c.h.c.web.sys.CubaDispatcherServlet - Initializing Servlet 'dispatcher'
2020-03-22T18:18:57.410837+00:00 app[web.1]: 18:18:57.410 INFO c.h.c.web.sys.CubaDispatcherServlet - Completed initialization in 836 ms
2020-03-22T18:18:57.420440+00:00 app[web.1]: Mar 22, 2020 6:18:57 PM org.apache.catalina.startup.HostConfig deployDirectory
2020-03-22T18:18:57.420441+00:00 app[web.1]: INFO: Deployment of web application directory [/app/deploy/tomcat/webapps/ROOT] has finished in [6,355] ms
2020-03-22T18:18:57.420992+00:00 app[web.1]: Mar 22, 2020 6:18:57 PM org.apache.catalina.startup.HostConfig deployDirectory
2020-03-22T18:18:57.420992+00:00 app[web.1]: INFO: Deploying web application directory [/app/deploy/tomcat/webapps/app-core]
2020-03-22T18:18:57.482880+00:00 app[web.1]: 18:18:57.482 INFO c.h.cuba.core.sys.AppComponents - Using app components: [com.haulmont.cuba]
2020-03-22T18:18:57.488898+00:00 app[web.1]: 18:18:57.488 INFO c.h.c.c.s.AbstractWebAppContextLoader - Loading app properties from classpath:com/company/cuba71sample/app.properties
2020-03-22T18:18:57.489683+00:00 app[web.1]: 18:18:57.489 INFO c.h.c.c.s.AbstractWebAppContextLoader - Loading app properties from file:/app/deploy/tomcat/conf/app-core/local.app.properties
2020-03-22T18:18:57.493873+00:00 app[web.1]: 18:18:57.493 INFO c.h.cuba.core.sys.AppContextLoader - DbmsType of the main database is set to postgres
2020-03-22T18:18:57.499788+00:00 app[web.1]: 18:18:57.499 INFO c.h.c.c.s.e.DataStoresCheck - Checking connection to data store _MAIN_
2020-03-22T18:18:57.827182+00:00 app[web.1]: 18:18:57.827 INFO c.h.c.c.s.e.EnvironmentChecksRunner - Environment checks on core module completed successfully
2020-03-22T18:18:57.892272+00:00 app[web.1]: 18:18:57.892 INFO c.h.c.c.s.p.PersistenceConfigProcessor - Creating file /app/deploy/tomcat/work/app-core/persistence.xml
2020-03-22T18:19:01.958554+00:00 app[web.1]: 18:19:01.958 INFO c.h.c.c.s.CubaThreadPoolTaskScheduler - Initializing ExecutorService 'scheduler'
2020-03-22T18:19:02.062120+00:00 app[web.1]: 18:19:02.061 INFO c.h.cuba.core.sys.jmx.MBeanExporter - Registering beans for JMX exposure: [app-core.cuba:type=PersistenceManager, app-core.cuba:type=ConfigStorage, app-core.cuba:type=UserSessions, app-core.cuba:type=UniqueNumbers, app-core.cuba:type=Emailer, app-core.cuba:type=EntityLog, app-core.cuba:type=FileStorage, app-core.cuba:type=LockManager, app-core.cuba:type=CachingFacade, app-core.cuba:type=ClusterManager, app-core.cuba:type=ScriptingManager, app-core.cuba:type=ServerInfo, app-core.cuba:type=Scheduling, app-core.cuba:type=PasswordEncryptionSupport, app-core.cuba:type=JmxNodeIdentifier, app-core.cuba:type=JmxLogControl, app-core.cuba:type=ClassLoaderManager, app-core.cuba:type=StatisticsCounter, app-core.cuba:type=BruteForceProtection, app-core.cuba:type=QueryCacheSupport]
What you can & should probably do is to compare the two projects and see where the delta is in the files.