timeFormat still can be defined either in message.properties or as a constant value in screen descriptor/controller (see demo). In order to have 12-h mode, you need to define an independent timeMode="H_12" attribute.
Thank you for the video with detailed explanations. The problem is that timeFormat can’t be used to change timeMode to 12-h, i.e. you can’t use a in your timeFormat. As I mentioned above, for the 12-h format, you need to define timeMode="H_12". But you still can use timeFormat to change time format, e.g.: