This seems to be a strange 10.0.173 bug. I have about 700 CDs ripped to WMA Lossless. I use a Pronto with Supernudelist to select albums to play. As a result, I added an AlbumID tag to all my WMA files. Everything worked perfectly. Unfortunately, my system drive died recently, and I had to rebuild my system. Thankfully, my WMAs were on a different drive, so I didn't loose any of them. When I rebuilt my library is where I noticed the problem. After I was up and going again, several hundred WMA files had their AlbumID tag cleared. Which files were affected seemed to be random. Some albums had one or two WMA files with missing AlbumID, some had only one or no WMA files with an AlbumID value. The AlbumID tag was there, but the value was cleared. It took me quite a while to get all the IDs back in there.
Obviously, this is not something that would happen every day, but I was a little nervous to see that tags had been tampered with.
Precise series of steps I took:
1) Rebuilt the OS (XP Pro)
2) Installed 10.0.173
3) Imported my files from D:\Media
4) Realized I didn't have AlbumID in the library, so I added it.
5) Updated MC library from WMA tags
6) Built AlbumID/Album view for use by Supernudelist
7) This is when I noticed all the files without an AlbumID. Out of 8000 files, close to 1000 did not have an AlbumID tag value. The tag was there. It was just blank.