I've searched through the forum and didn't see this bug reported anywhere.
Using the Windows Store version of MusicBee.
Using a Rainmeter with a CAD plugin, for whatever reason album art would not display in the plugin for Track 1 of any album. Track 2+ work fine. This only seems to be when the Album Art is embedded. If its cover.jpg it seems to work fine.
Rainmeter is looking for the album art dat file located in:
C:\Users\NAME\AppData\Local\MusicBee\InternalCache\
but that doesnt exist in the store version because its located in
C:\Users\USER\AppData\Local\Packages\50072StevenMayall.MusicBee_kcr266et74avj\LocalCache\Local\MusicBee\InternalCache\
what i ended up doing as a workaround was symlinking from Appdata\Local\Packages... to Appdata\Local\MusicBee and it all works.
Not sure if this is a bug with the rainmeter, cad plugin, or MB.