Hello,
Thanks to the help of a user, the “thermostats not showing up” issue has been root caused and even has a work around for now until a real fix is pushed to Apple.
In a nutshell, your thermostats needs to be assigned to a room to be visible.
This was never caught during our beta testing but for now, simply moving your thermostat into a Room in UI4 should make it appears again in the Thermostats tab. We are working on fixing this issue for next update.
Sorry for the inconvenience and thanks for the users who contacted us to help fixing this problem.