This finally works with iOS 15 beta 1. As soon as the new model version is installed on the second device, it got all the changes populated in it. Thanks a lot for fixing it!

Created FB14138815 for this issue.

At least with Xcode 16 beta 5 it is possible to add the package as a local dependency to the demo project. The following steps give me the result I need: Open Project > Package Dependencies > Add package > "Add local" Optional for more convenience: Add package as reference: Drag&Drop folder of target from the package into Xcode 1. Action: Reference files in place 2. De-select target