I upgraded the YT Music app the other day, and now it won't play music unless the app is active (i.e. no background play).  I've done the usual - signed out and back in, force quit the app, restarted the phone, and confirmed I have YouTube Premium.  It appears to be a bug in the app, which I've reported to YouTube, but I'm interested if anyone else has experienced this problem (acknowledging I'm probably one of the very few using YT Music instead of Spotify).