Airplay volume jumps

Using Music App on iPhone to Airplay to different speakers…

1) the playback volume jumps to around halfway on the volume slider every time use next track / track forwarding.

2) when several tracks have been playing and the interface hasn’t been touched for a while, the volume suddenly increases as soon as you touch the volume slider.

Fault appears on several different speakers

Fault not apparent when playback is from iPad Pro on 17.1.2