Playlists not syncing between devices

This is a weird problem that has only recently started. I am having no trpouble syncing downloads between my apple devices. So if I download a song from Apple music on one device, it will show as a download on all my devices. The sync checkbox is ticked on all my devices.

However, when I try to add a song to a playlist, the playlist does not sync, so the added song does not appear in the same playlist on any other device.

If I click on 'update cloud library' in the file menu, the playlist does not update on other devices. Worse, the disappears from the playlist it was added to on the first device.

So syncing is working for any music downloaded from Apple, but syncing is not working when I add that song to a playlist. It seems like adding the song to the playlist doesn't register with Apple's cloud server, so that when I try to update the cloud library, the server thinks that the song I added to the playlist shouldn't be there, and deletes it from the playlist.

Any help appreciated.

I should add, OS is up to date on all devices.