My 2 cents - I am having the same problem - my mp3 files show in the Advanced
Settings in the Summary Tab only 3 items => Track Number, Title, and Comments.

I no longer have Artist, Album, Genre, Bit Rate, etc. It is just gone!!! I
previously had info there. Luckily WMP 10 Advanced Tag Editor saved the info.
However, in Windows Explorer I cannot any longer Group my files or arrange my
icons.

I have been trying to fix a problem with WMP 10 and noticed this. This has
affected all of my media files. I have rolled back WMP to 9 and then upgraded
back to 10, and reinstalled DirectX 9.0c to no avail.

Unfortunately, I do not have an answer but maybe something to think about.
If you solve this mystery, please post what you did. I shall do the same if I
find a solution.