First, it's a great program especially the scripting part.
Second, I found a problem in version 2.38. I tried to 'renumber' the track numbers with leading zero's. In MP3 files it works very fine, in MP4 files it seems to work, but I don't see the result on the display. If I look at the file with, say AudioShell, I can see the track number has a leading 0 but it doesn't display in MP3TAG. Tried to close and open MP3TAG, but stil not displayed. I also tried to make the field alphanumeric (removed the tick in Columns) but w/o success.
Edit: just to be sure I installed the latest build (2.38a), but w/o success, problem still there.