Minim not passing through embedded artwork?

[windbag]

Setup for info:
I’m using Minim(free) after recommendation by Bubblesoft uPnP developer, to pull QNAP NAS music files to an older (S1) 9-unit Sonos system. Basically working with some issues.

Music is limited to mp3 files, artwork embedded in the file, generated by extracting from CD using EAC v1-6, but replacing the default artwork with one that has been squeezed down to around 30kB, so the inclusion in every track is not too heavy on the overall size. All music organised as:
Band or Artist/sound files/_studio/albums by recording date
____________________/live/concerts by play date
___________/playlists
Band Alumni/first member/sound files/ etc

All playlists are relative, so that chunks of Band (+Band Alumni folder pair if relevant) can be moved in the overall file structure if needed.

Big emphasis on playlists, as playing albums can be done by just navigating to the base folder & playing.
Minim kindly got my playlists to be recognised, but am having a problem with artwork not appearing in the playlist view in Bubble. This is important, as it can be the quickest identifier of which version of a track is appearing in a particular playlist.

.. .. Have liaised with Bubble developer; from me:
Testing out the playlists... of the 13 tracks in the list, only two showed the correct artwork.
I went and checked the files - all my mp3 tracks have artwork (~30kB size) embedded in them, and all 13 did, created in the same way.
{I checked the artwork in each using Mp3Tag}
.. .. Got this response:
It is a problem of the media server indexing the artwork and exposing it to the app (as a http image URL). It might be a MinimServer issue, not recognizing some of the artwork for some reason.
You can try to trigger a rescan (can be done in MinimWatch). If the problem persists, it is best to ask for help in the MinimServer forum.

I have done the suggested re-scan - made no difference

So, here I am asking for suggestions why track-embedded artwork done via the same route might only be intermittently "exposing it to the app (as a http image URL)"