Normally you can unmute a muted single room and also a muted group of rooms by moving apps volume slider. 
Latest Android app for that works like expected, but latest iOS app doesn’t and I suppose it’s a bug. 
 
Using volume slider for a single room or main slider for a group of rooms doesn’t work. Just using the room slider of a room that is part of a group will unmute this single room. 
On Android it works in any case. 
 
Here‘s a video of my iOS app (latest version) on iPhone 14 (latest iOS version). 
Using iPad 10 it’s exactly the same. 
 
