iTunes 12 album art won't save
I've got iTunes 12.8.0.150. I'm trying to save album artwork for two albums I recently added by importing the CDs. I import my CDs to mp3 tracks. I've tried adding the artwork using Get Info > Album Art > Add Artwork > OK, but it disappears when I click OK. I've tried quitting iTunes and restarting. I've tried adding artwork to a single track. I've checked file permissions to make sure I have read/write access. I've tried dragging and dropping the album art. Nothing works. Any ideas?
MacBook Pro, macOS High Sierra (10.13.6)