trott,
Based on what you said, I guess the problem is unrelated to the m4a changes. Did you experience any crashes? Is the library in the default location? If you are using Library Server have you set the clients to update changes back to the server?
One thing that you can do is to restore the latest "good" library backup file. It should bring the missing field values back.
Actually, if you have not done what Jim advised, you are probably still using DirectShow decoding. The native m4a input plugin reads tags a lot faster, but it is still a bit buggy. HE-AAC playback doesn't seem to work correctly and seeking of ALAC files is not possible. Probably these problems will be fixed soon. However, even if you have files in either of these two formats, it is possible use the new plugin for importing and/or tag reading and after that switch back to DirectShow decoding if you encounter any playback problems.