Apple Music: "Item not available in your country or region" error in iOS.

Odd and irritating problem this. I have a large music library comprising lossy AAC, MP3 and Apple Lossless files, all stored on an external drive. These were added to Apple Music in macOS running Ventura 13.7.2 without copying the files over to my main Music folder and ensuring the lossy and lossless files were kept separate ('keep music media folder organised' is unchecked as I don't want lossless and lossy albums mixed up). I also subscribe to iTunes Match so that I can access my music anywhere without having to carry a hard copy of the library around with me. All is well and good for the lossy but not the lossless files. In the latter case, many (but not necessarily all) of the songs in an album won't play. Instead, they're grayed out and I get a "This item Item is not currently available in your country or region" dialogue when I try playing them on Apple Music in iOS in or out of the house. The curious thing is that the same files are available to play on the Mac housing my music folder (on the external drive) through the Music app and will play just fine in iOS over my local network using Airplay and the 'Remote' app. So this is a problem restricted to file access through the Music app on iOS. I have tried clearing caches, signing out and then back in again on multiple devices, clearing my entire library and then importing everything again (after deleting the locally stored musiclibrary file). But nothing seems to fix it. All my devices are signed into the one Apple account registered in the UK and I have no other. Any other suggestions for a fix gratefully considered!