Apple Music / iphone problems (M3 Sonoma 14.6.1 / iphone 11 pro with iOS 17.5.1) Library seems stuck

I have a problem .. When I open Apple Music, my computer (M3 iMac running 14.6.1) shows a full library of songs on my iphone (11 pro with iOS 17.5.1). However, the iphone has not got this music on. (If I look at 'storage' it is not there)

My music library is big (35,000 aac files I think) but the phone has capacity to spare for that.

This problem seemed to coincide with updating to sonoma 14.6.1. After getting flustered & I think deleting all music on the phone thinking to re-sync/re-transfer (only bought from Apple Music items were showing anyway), I saw that 'sync all' was turned off. Does 14.6.1 do this on opening??

So, I set to 'sync all' but .. the computer believes that the music is already there. It will not delete from the computer end

Is there a preference file or similar that I can delete that might clear this?

Can I uninstall & re-install Apple Music (I don't think this is possible?)

Any help, gratefully received

Thank you

(ps Ken Burns is innocent)