When a Display is assigned to a Zone, and the Zone is then grouped with another Zone to form a new Zone, the Display doesn’t follow the Zone to which it was previously assigned. The screen just goes to the splash screen.
The undesirable behavior here is that if I’m listening to music in the living room, with the living room TV assigned as the Display, and I want to hear the same music in the Kitchen if I’m making dinner, and I join the zones, the living room TV no longer displays information about what’s being played in the living room.
The desired behavior would be that when zones are grouped, all the displays assigned to any of those zones are assigned to the new grouped zone, and when they are ungrouped, the displays go back to displaying what is playing on the original zones.
(Another way to put this is that Displays should be assigned to Outputs, not to Zones.)