Windows 7 Professional, x64 SP1
Setting "Split multiple-value fields with:" using a non-standard value of slash: " / ", I'm not getting separated artists correctly listed for FLAC files.
At least I think that's the reason, because AFAIK, that's the only difference between the files.
Shown:
Result after adding an MP3 file and FLAC file with identical tags to the library (the MP3 file splits correctly, the FLAC file does not):

Attached:
The two sample files I am using (blank files with tags):
http://www.mediafire.com/download/4ikp2 ... t+Test.zip