This should be counted as a bug shouldn't it? FLAC does not save initial key tags as INITIAL KEY. The correct format is INITIALKEY.
It isn't the 'correct' format, nor is it a bug.
Vorbis Comment does not dictate how tags like these should be written.
It doesn't even have proposals or suggestions for it. (except for a couple of obvious ones such as ARTIST, ALBUM, TITLE)
So it's completely up to the software what to use.