I started this thread. The question was mine to begin with. Mine wouldn’t sync in my iPhone, my playlist disappeared, as did the sync toggle button.
I tried some things and MINE acted like I didn’t even have applemusic anymore. Which is my entire playlist and music collection.
So I found my playlist was still on my iPad. I shared it with myself. Then I updated & rebooted, hard reset and shut down and powered back up.
Many of my songs were greyed out. That started my whole mess. I don’t want everything of mine shared in the cloud, so that’s how my sync button disappeared. There was no ‘are you sure? This is what will happen if you press this button’ … so I hit the button, which took me to the DarkSide.
I hope you find resolution. I did and it was not easy at all.