As you can see in the following screenshot
Multiple entries in the "Remixer" Tag TPE4 are NOT handled correctly.
This file has been tagged with Picard.
The two artists with the "Remixer" role have been parsed correctly by MusicBee but the entries should show up as separate entries in the Artist Splitter and "Artist"-View in the Library Browser.
By the way, why is the name of this field called "MIX ARTIST" instead of "REMIXER" according to the spec
http://id3.org/id3v2.4.0-framesTPE4
The 'Interpreted, remixed, or otherwise modified by' frame contains
more information about the people behind a remix and similar
interpretations of another existing piece.
Picard at least tags the mixer roles as "Artist:mix" as part of IPLS (ID3v2.3) and remixer go to TPE4, as you can see in the screenshot.
There is another oddity here as you can see in the next screenshot:
When I make a dummy change in this example, and update and save the tag information via the "Multiple Artist Splitter"
MusicBee writes the artists flagged as "Remixer" as an additional tag "TXXX/REMIXER" explicitely called "REMIXER" in the Tag Inspector.
I'm also missing the entry of "Artists:Remixer" in "Choose Categories..." in the Library Explorer. It's there in the "Set Displayed Fields..." Menu in the "Album and Tracks"-View