iPhone won't play some songs synced from Mac

This is driving me mad and seems to be getting worse! I searched the forums and found that lots of other people have had similar issues, but nobody seems to have found the answer, and all the suggestions offered (e.g. restart the phone, sync it again, make sure you've installed the latest updates) are useless, because of course I always do those things anyway.

I am a musician and I have a very large number of tracks in my Music app (formerly iTunes) on my Mac. I use smart playlists (and many tags) to manage them, but they are all physically on my laptop (and on backup disks), NOT in the cloud, and have been either ripped from CDs years ago or bought from iTunes as mp3s. (I do also pay for Apple Music, but I don't use it very much.)

I don't sync my entire music library to my iPhone - just a subsection of it (specific playlists that I need for performances or teaching etc). But in the past few months I have started to get the error in the screenshot below. The song still exists on my laptop, but on the iPhone it is greyed out and won't play, and subsequently it vanishes from the phone entirely, and syncing again doesn't replace it. So there is now an increasing number of songs which I own (physically) that I can't play on my phone, and it's reaching the point where there isn't much point using the phone in my work at all, because I have to have the laptop fired up anyway for those songs. (These are all songs that I've had for years, and played many times on the phone before, but otherwise there is no obvious connection between them.)

Anyone got any bright ideas, please?

(iPhone SE, iOS 16.6, MacBook Air, macOS Ventura 13.4.1... I know there's an update available today, I haven't had time to install it yet, but I will)