Actually this was already mentioned by DetLevD, but on the Offtopic forum, here:
Empty tag-field in "FLAC / Vorbis Comment" tag?
Fortunately I managed to find this before posting it, but I think this thread should be moved to here, because it's clearly a buggy behaviour.
Since the description is there, I'm not going to repeat myself here again.
I would consider this bug to be nasty and relatively important to fix, although not critical.