I noticed that the volume control via side buttons is not very intuitive and even buggy. Let me explain. I use Spotify as the player but others are affected as well.

Music playback on Pixel speaker while screen unlocked
- Volume up/down in/decreases volume
- volume bar pops up and moves
- continuous pressing in/decreases volume up to min/max
Music playback on Pixel speaker with locked screen that is off
- Volume up/down in/decreases volume
- volume bar does not pop up ->counter-intuitive!
- continuous pressing disabled, you must press repeatedly ->counter-intuitive!
- Music playback on Pixel speaker with locked screen but ready for unlock
- Volume up/down in/decreases volume
- volume bar pops up and moves
- continuous pressing in/decreases volume up to min/max
Music playback on Spotify connect while on app
- Volume up/down in/decreases volume
- app-specific volume bar pops up and moves
- continuous pressing in/decreases volume up to min/max
Music playback on Spotify connect while on other app
- Volume up/down in/decreases volume
- volume bar pops up but does not move ->bug!
- continuous pressing in/decreases volume up to min/max
Music playback on Spotify connect while screen locked and off
- Volume up/down in/decreases volume sometimes but sometimes not. ->bug
  Theory: After hanging up a call it does not change volume. Normally it does.
- volume bar does not pop up ->counter-intuitive!
- continuous pressing disabled
Music playback on Spotify connect while screen locked but ready for unlock
- Volume up/down in/decreases volume
- volume bar pops up but does not move ->bug!
- continuous pressing in/decreases volume up to min/max

I tested the Revox S235 app as well. Interestingly there the volume bar pops up when screen is locked and it moves. But unlike Spotify connect it does not change volume. Which is twice counter-intuitive because it is different than Spotify and what you hear is not what you see.

So is this an issue of the apps or is it a GOS issue or even both?