A temporary fix that has been working for me for months:
1) make sure the music that is playing on Apple Music is playing only through iPhone or iPad - which ever you’re using. So, make sure Airplay is turned off.
2) Close Apple Music Down on iPhone or iPad
3) restart Apple Music
4) turn airplay on
volume control should be available until you do one of the following:
a) turn airplay off and then attempt to restart airplay again without closing Apple Music down FIRST with airplay turned off
b) pausing music for a long period of time. For me, anything beyond 10 or 15 minutes and the volume control disappears
if you lose the volume control for any reason, make sure you turn airplay off, playing music only through iPhone first, then close Apple Music down, then relaunch and start again. If you close down Apple Music while Airplay is engaged, the volume control will not return when you relaunch Apple Music. The key is to make sure Airplay is off before you close down Apple Music.