Cannot control volumes separately on Airplay 2 since latest iOS upgrade

I used to be able to control the volumes of speakers when streaming music to two or more Airplay 2 devices for each device separately. With the latest update this possibility seems gone and both speakers are shown as a group with a single volume control in the control center.

How can I ungroup them?

Needless to say that this grouping does not make sense as you want to adjust volume on one device but not the other in separate rooms.