Some additional observations regarding this topic…
After the latest update of ARC, and following a suggestion made by (@)Suedkiez here, I scrolled through all my albums in ARC. First impressions were good. Artwork was systematically displaying correctly.
I then experienced a situation in which I had poor mobile connectivity. During this time, I stopped playback as I was existing the car, and restarted when I returned. As I mainly listen to downloaded tracks while using ARC, playback of my tracks was never interrupted. However after resuming playback, album art was again not showing.
So it seems as if caching on the one hand does indeed improve correct display of artwork, but it does not seem to eliminate it.