I've been having this issue, too. Thousands of songs are now showing "unknown artists" and "unknown albums". I've tried just about everything, sometimes it'll fix itself, sometimes it won't. All the info is there when I look, some songs allow me to edit, save and then it'll show everything correct and others just tell me I need to grant access but every time I do that it just asks for it again.
A huge part of my library has been completely screwed up.