I was lucky enough to spot the sorting order issue cause by Picard soon enough, it was easy to fix.
You can set some options in Picard in the “Scripting” section. I created 2 scripts, the first one:
$set(artistsort,%artist%)
$set(albumartistsort,%albumartist%)
which sets the sorting order to the Album Artist, which is usually FirstName_LastName.
That worked well, until I realized that it would set the sorting in Plex to “The Beatles” instead of just “Beatles”.
So I disabled that script, and created a new one, that removes the sort tags downloaded from MusicBrainz before they are written to the file. Seems to work properly (for now 
)
$delete(artistsort)
$delete(albumartistsort)
Try them and see which one works best for you.
