Interesting. Sounds like a minor bug, but I'm really not sure, as you are not allowing MC to write to your files.
For me, I'm mostly "all in" on MC as a music manager. I no longer use external tools because MC does almost everything I could want in tagging or adding art work. I also explicitly tell MC to write to my files because I want the tags to be in the files so that everything I do in MC (tagging, etc) is available when I move my music files to other systems.
I trust MC to do the right thing with my files. I have 99.9% FLACs and MC handles them quite well (tags).
I'm guessing you had a bad experience and don't trust most taggers, so you only use MC for playback. You probably also have a tagger that you REALLY like and don't want to change. Both are understandable. But you are creating a bit of an edge case here.
I think MC is one heck of a tagging program (very powerful, fast, and repeatable). But even if you don't want to use it for tagging, you should probably allow it to write to your files. That might solve this issue and prevent future ones.
Just MY opinion.
Brian.