Hello @Flumi
This isn’t actually a bug, but the intended behaviour of the field:
That field isn’t made to store timezone information, only a date and time.
So the timezone dropdown only shifts the currently set Date-Time to that specific selected timezone. Once saved, only the changed Date-Time is saved, with the shifted hours, but not the actual label to the timezone.
I can see how this can be a bit confusing, but it is necessary to maintain the same time-zone throughout the whole project: If you go to /admin/environment you can see the selected global timezone for your project, that is the time-zone applied to every Date-Time field, consistently throughout the project