Music synced from desktop iTunes doesn't appear in iPhone Music app, despite being there in storage

Hello all, I use the music app on my iPhone SE just as an offline music player; thus, I've disabled all Apple Music and iCloud features for the app. I sync the music to my devices from iTunes on my windows laptop.

Today many albums I synced a couple days ago from iTunes just disappeared from the music app, despite being on the device when checking from settings. Searching for them inside the app leads to nothing. Many albums disappeared completely while some retain only a few songs.

What's strange is I already had this problem with my 2016 iPhone SE a year ago, but it was worse because only a limited and fixed number of songs would show up in the music app despite all being correctly synced. That time it was an iTunes library corruption problem, and I fixed it somehow but I don't remember.

I've tried all the most common fixes and even reinstalled iTunes and deleted the library .itl files completely thinking the library might have been corrupted again, but to no avail. I even converted the song files that would not appear from .m4a to .mp3, even changing metadata of the songs. Maybe I could rip or download again all the songs that don't work, but it'd be the last option given their number.

The weirdest thing is that using another third party tool to transfer the songs to my device they show up. But syncing again with iTunes makes them disappear again from the app. So it must be either an iTunes problem that somehow doesn't allow the songs to be visible in the music app, or the app itself on my phone that is bugged. I've reset the phone two times already and set it up as a new phone, no change.

What could possibly be the culprit?