Downloaded apple music disappearing from iPhone 12

Same/similar problem for me. I have an iPhone SE with loads of storage and Apple Music subscription, with a few thousand songs downloaded. I had to sign out of my Apple Account for an unrelated reason and after signing in again, all my downloaded songs, except those in a two random playlists, appear to be no longer downloaded. They are all there in My Library, but not downloaded and so unavailable when offline. I've tried the various solutions I've found in other threads on this issue (which all seem to be several years old/out of date, or for music on a Mac), but they haven't worked. I also completed the latest update, 18.2.1, and it made no difference. Really hope there is a solution to this because I don't want to have to go through and download all the songs again, especially given scott99lee's experience it sounds like they might just disappear again.

Apologies for not helping, but I have also had a similar problem. I use an iPod touch and suddenly one day a couple of weeks ago all my downloaded songs had disappeared. Obviously an iPod doesn't have data so I needed the offline copies. I tried to resync connecting by lightning cable to my Mac and it wouldn't sync (couldn't find iPod). I ran a wireless sync and it resynced most of my library but not all. There are loads of random missing tracks. So an album might have all but one or two tracks synced. If I try to play the unsynced track I get an error saying 'this song is not synced across your devices'.

In addition, any albums which are not recognised by Apple Music are not synced at all.

Lastly, this is not a device-specific issue. I have synced to my iPhone SE as well and I get the same issue.