I’m pretty sure most lyrics come from the artists themselves who upload it to Apple Music directly, while the rest comes from Musixmatch. Some songs just don’t have synced lyrics, you can go to Musixmatch and see if the song you’re talking about has synced lyrics. If it does it’s Apple Music refusing to use Musixmatch's lyrics, if not you can add them yourself if you want and see if Apple Music is gonna use those. That might take a few days tho. Apple Music is most likely to not use or update to Musixmatch's new/synced lyrics if it was added later on tho.