Music star ratings not syncing, and disappearing

Hello. I'm experiencing some irritating and inexplicable behaviour with star ratings for songs in Music.

For context, on all my devices I have 'Show Star ratings' turned ON in Music settings, and I also have the box beside 'star ratings' ticked in Music > Preferences > General on my iMac. So I am not looking for this advice.

When I am listing to music on my iMac, and give a song a rating, ten minutes later all the songs I have rated in that period no longer have any ratings. When I add a rating to a song on my iPhone or my iPad these sometimes remain, and sometimes they disappear too when I look at my 'recently played' playlist on my iMac.

I used to be able to ask Siri to add a rating to a song, but this function either has been discontinued, or has stopped working - Siri now tells me that is something it can not do (it always used to).

I have a Music subscription, and I log in to all my devices using the same account. Playlists do sync, but not immediately - the same songs will play over an again in a playlist set to only play songs that have never played - so there is something not quite right across the syncing board.