I started encountering this a while ago, just randomly (
post here), and I'm honestly not sure what could have caused it.
I didn't make any changes to my wine prefix (I do have a dedicated prefix for MusicBee) or my Wine install since I started using it. I'm not on staging, and the only change that was made was updating to 10.0 at the start of the year (though I started encountering this before this update)
The only thing that really happened was I was converting all of my WMA tracks to Flac since I could't get WMA playback work, something happened at that point, and now no album art loads at all in the main view.
I have a hard time believing it's just a Linux issue, because it was working perfectly fine at first, but then something changed and it suddenly just wasn't working after that, and I have no idea what could have triggered it.
I guess some other info, I use EndeavourOS with the 6.13.1 Arch kernel and KDE Plasma 6.2.5, and I use the Wine 10.0-1 main package from multilib and a 32-bit prefix for MusicBee.
This is pretty much the only problem I've encountered with MusicBee on Linux (aside from no global hotkeys or media integration but that's more a Wine problem than anything else) so I'm a little confused at this obscure issue.