Another solution that worked for me was to simply turn on notifications for spotify and it popped up on my lockscreen again 
Hello everyone! I had the problem of YouTube Music controls not appearing on the lock screen, but none of the other solutions listed on this thread worked for me. I fiddled with other settings and found something that worked for me. If you're still looking for a solution to this problem, try this:
1. Open the Samsung Music App (even if you don't normally use it to play music). It should be installed by default on your phone. If you can't find it, then go to the Galaxy Store on your phone, and open the app from there.
2. Go into the settings and toggle off and then on again these two settings:
- "Control music from lock screen" (turn off then on again)
- "Allow external devices to start playback" (turn off and then on again)
This worked for me to get YouTube Music to appear on the lock screen, and I hope it works for you!
I spent MUCH longer on this than I anticipated and scrolling through this thread I tried almost everything including updating the lock screen (I'm new to android, didn't know it needed to be updated!)
As the saying goes the easiest solution is the best and so I present proof that I managed this (it is a Very handy feature to have)
In Spotify settings there is a Broadcasting button, make this your FIRST check, it could save you 30 minutes (i know I know..)
For reference, a screenshot of the other setting as well.
That's what finally made it work for me having already looked at the other settings. Had no idea tapping the clock would show widgets. There was a generic-looking music widget there and when I tapped the "play" triangle it turned into a Spotify one
