Actually, the ideal ID3 tag for ALL Sansa players is ID3v2.3 ISO-8859-1 (which is actually different from ID3v2). Having said that though, FLAC files have their own tag format.
I think you have your MP3Tag settings misaligned. This is how mine are set-up and I have never had a problem with FLAC files (or MP3) out of the (literally) thousands of files I have ripped and transferred to my various players:
READ:
ID3v1 (checked)
ID3v2 (checked)
APE (unchecked)
WRITE:
ID3v1 (checked)
ID3v2 (checked)
APEv2 (unchecked)
ID3v2.4 UTF-8 (unchecked)
ID3v2.3 UTF-16 (unchecked)
ID3v2.3 ISO-8859-1 (checked)
ID3v2 only if ID3v1 too small (unchecked)
REMOVE:
ID3v1 (unchecked)
ID3v2 (unchecked)
APE (checked)
If your FLAC file tag settings aren’t right (and aren’t being read by the player), it’s due to a wrong setting in whatever ripping software you are using. Hope this helps. 
