Tried that but no luck either. Interestingly enough is that that the LOV does have SETSUBTITLESUBTITLE, which seems to indicate that it somehow combined both SETSUBTITLE and SUBTITLE.
I'm seeing this too in the mp3tag.cfg when using the Save configuration... option:
RATING WMP RELEASETIME SETSUBTITLESUBTITLE TAGGINGTIMETITLE TITLESORT
Yes, the problem is my current installation because a fresh install doesn't have this problem. The only 'reset' option I came across is Tools --> Restore all Input Fields but that doesn't fix it. Neither one of them ever was shown as column so I'm still curious to know how Mp3tag ended up gluing those two tags together.