I have just had the same issue.
My iphone was wiped for a screen/battery replacement (was on beta software) and after reinstalling everything again I connected to my mac to transfer all my music (38gig) and all album art was missing.
A week later it was still like this so I deleted the music app on iOS, deleted all music on the phone, reinstalled the app and re-synced all music (with proper album art on the mac btw) but again nothing. Except for 3 songs I once gave a <3, and the iTunes purchased songs, I have no album art.
I downloaded another couple of music player apps ( Doppi was the best one ) and here I get a hand full of albums with artwork on some of the songs, mostly albums I have once uploaded the artwork myself I believe.
So weird...
iPhone 13 mini on iOS 26.2 (no longer bèta)