sync music on two devices, only the first one updates

I have my Apple Music (formerly iTunes) library on my Mac Mini. I sync some of the music to my iPod 7, and all of it to my iPhone, which I upgraded a few weeks ago to an SE (2020). All devices have the latest software updates. Wherein lies the problem.

I have smart playlists that use Last Played as a trigger, because I want to rotate songs on the iPod. So I sync my devices and update the playlists pretty much on a daily basis. Aside from some major tragedies caused by a MacOS update last fall, for the last few months this has been working well.

Last weekend, when I synced the iPhone SE, the Mac gave me a message that in order to complete the sync with the SE, it had to install a MacOS update. This was a few days before the big update to 10.15. Since that little update installed, I can only sync one of my two devices. Whichever device I update first, and I've swapped them around, that one will update. The other one goes through all the motions, no error messages, but the Last Played values in Apple Music do not change. I've played five or six hours of music on my iPod, and the Mac is like "Oh, did you do something?"

I have rebooted the Mac (also at least daily, due to the aforementioned tragedies), and rebooted the devices, between attempts to sync. I have tried disconnecting the devices and restarting Music after a Full Quit, but nothing is changing this. This morning I tried syncing both at the same time, but the iPod finished first, and it won the race. The hour of music I played yesterday on the iPhone didn't change on the Mac at all.

And if you're wondering, why do I need two devices? Portability, and sound effects. When I'm playing music, I don't want to hear the notification tones from the iPhone. Also, I can leave the iPod in one place, and walk away carrying the iPhone, and the iPod will still be waiting for me when I get back.