Hello Stas – were you able to resolve the problem? I had a similar experience… airplay volume as played through Apple Music etc. worked fine, and inexplicably stopped working after, and honestly, I don’t remember if there was even an update, but I think there was as I remember rebooting the device and the volume control was gone. I have tried setting the volume settings as you have, but there’s no change, iOS volume just doesn’t work anymore. I wonder if it’s a setting change, or if an update of alsa or some other linux service has broken the volume control.
Edit: apparently setting a zero volume on Apple Music (MacOS, iOS) works to mute the music, but any other change doesn’t work.
Edit #2: since I am digging around on this issue, I have found another user for whom volume control IS working as of January of this year. Link to the forum post is Ropieee volume issue - #75 by DanMtsn