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.