As far as I can tell, for any given release I have, if it requires the change of only the cover artwork type, then it thinks it saves OK afterwards, but removing and adding it again shows that the exact same change is required again.
This is definitely an issue for me using the above noted versions of Picard in the above noted OS. I'm not sure if it's specific to just these or others too, sorry.
For example, I can import David Bowie's Low. It requires the cover art to change from "Front" to "Front, Poster", as revealed by the "Show more details" button. It seems to save OK. I then remove and re-import it, and it notes the same required change.
I've attached a debug log.
Thanks, and keep up the great work!