I don't have a deep understanding of the structure of the program so tags etc are an unknown aspect.
All I know is that there is a Cover Art file within the J Rivers MC 16 suite and when I have to find the suddenly disappeared album Cover Art, it is usually there.
If it isn't, I rescan the CD cover or search the internet.
I have previously tried MC 18 and MC 19 and today, MC 20.
In every case the automatic search for my music files (over 13,000 - 566 GB) puts many of them in an Unassigned category and thus renders the program useless.
These later MC versions do not replicate the complete and satisfactory presentation of MC 16, achieved with some patience getting multiple artists to appear under one album cover (and not a separate one for each track!) and where (for example) 5 tracks of Brahms Piano Concerto No 2 are each shown under an individual album cover!
With MC 20, when I tried to reinstate just one Cover Art as a test, even having changed the Tools/Options/Cover Art to point at the MC 16 suite, where they are, MC 20 persists in prompting for the path each time.
I accept now that I can never move away from MC 16 and will live with the sporadic disappearance of Cover Art.