iPhone Music app keeps deleting random music tracks

This has been irritating me for a while now. The Music app on my iPhone 13 Pro keeps deleting random tracks from some of my albums. These are not ones I have purchased from the Apple Music store, they are albums I have manually added to the Music app on my MacBook which I have synced to my phone. They will all be there for about a week and then when I check back a handful of tracks are no longer available. They're not sequential tracks either ie: 1-5, it's tracks 1, 4, 6, 7, 9, 14.

I've been syncing my phone with my laptop almost every week to get these tracks back on but, my god, it's annoying!

I have plenty of space on my iPhone (over 100GB free) so it's not a storage issue, and my phone is up to date with the latest iOS.

This was happening before I subscribed to Apple Music. I now have a subscription and if I turn off Sync Music Library on my iPhone then some of the missing tracks miraculously come back, but then that means I've lost access to my Apple Music subscription and any music I've downloaded from there are instantly deleted. I also have to switch off Sync Music Library otherwise I cannot sync my iPhone with my MacBook. Once I turn the function back on on my iPhone I'm back to square one again with random tracks disappearing a week or so later.

I am not bothered about syncing tracks from my Apple Music subscription with all my devices as I rarely play music on my MacBook.

Help me! It is driving me nuts!