UPDATE: I've noticed that adding new music and updating normally recreates a lot of missing album art. If I SSH into the device and manually remove all of the converted thumbnails, then reset the Bryston DB and let it rebuild... I then see all of the artwork (except for the issue noted above).
OK, I've separated my multi-disc albums into separate folders, so:
Album
01-01
01-02
...
Album [Disc 2]
02-01
02-02
...
And my album art issue seems to be resolved; I have cover art in all of the multi-disc albums with Manic Moose.
However, the problem quoted above is still there: I need to manually wipe all of the bdp_front_*.jpg, et al. files and then Update/rebuild to get 100% album art. If I just add some new music to my collection, restart and click Update, it updates, etc., but I am missing a LOT of artist thumbnails in Artist View. In other words, the bdp_front_44 files; the larger bdp_front_250 files still work and they are all present when you click on a particular artist.
Finally, when I check manually (i.e. SSH), both sets of thumbnail images are present on the filesystem.
For what it's worth, I've even cleared my browser cache, but nothing works except to wipe the thumbnail images and rebuild.
S2.29 2016-09-20
Build: Manic Moose
MPD: 0.19.12 NEWS
Kernel: 3.16-0.bpo.2-486