I don't believe that's the case. Why do you think so?
What filetype? FLAC? If so, please see the thread on FLAC problems.
The files are regular MP3's. Not the case? Well, that's certainly possible but neither mp3tag, foobar2000 or dbpoweramp can read the tags after updating them from MC13 if they originally were id3v2.4 (again, there are never any problems if the files are id3v2.3 or lower before trying to tag them).
Now, I tried to check this further using yet another tool, this time TagScanner 5.0 and it also had problems with the files after they've been updated using MC13, but it did show something that the others didn't. It claims the MC13-tagged files now have id3v2.4 utf-16 (they all were utf-8 originally) tags, despite the fact that I don't think Media Center in any edition have been able to write anything above 2.3 (am I wrong here?).
So it appears not all information is completely gone, but MC13 clearly have messed up something it shouldn't have...
EDIT: Tested some more, and things goes a little better if I remove all embedded artwork before reloading information from the files then re-tagging them. It still messes up a bit, like I now end up with duplicate "year" frames along with the fact that the encoding of the tags have changed, but at least now other programs can extract the tag information that remains...