I've had it happen with a very small amount of music (1 to be exact) that I updated using Windows Media Player outside of Media Center, so it isn't just stuff you've updated manually yourself. It also seems determined to occasionally change information on a very limited number of my songs, even after I've fixed them previously. It's not enough to make me give up on keeping my music organized, but it is frustrating.
Vlad
HTPC: Intel D945PVS mobo, Liquid Cooled 3.0 GHz Pentium D, 4 gig ram + 4 gig Ready Boost, Windows7 Home Premium 64 bit, 3x300GB + 1x1.5TB hard drives, Origen X15e black case, HDHomerun + 2 x Avermedia A180 (total 4) QAM tuners, Hauppage WinTV-PVR-500 dual tuner analog card, Lite-On 1693 DVD R/W driver, AnyDVD, Samsung UN55B6000 55" LED backlit LCD flat panel, TOSLINK to 5.1 surround sound