I've had the same problem.
It's seems Jaikoz is screwing up the tags. I know i've trusted Jaikoz too
But after I get mp3's into mp3tag, select them all, do a "tag cut" and then do a "tag paste" it fixes them. It also makes them smaller in size if they have large gaps from large artworks that had been deleted from them.
You'd have to wipe wmp's library database AND the artwork cache for the re-scan to be sucessful.
So my workflow now is:
mp3gain - remove gain info
jaikoz - sort every detail out
mp3tag - fix jaikoz's stuff up's
mp3gain - apply album gain to all
wmp - scan without volume levelling
Too many steps if you ask me but it seems to work well.