This seems to be recent, though I can't pin down the age.
When I open a "bunch" of mp3s into MP3TAG, using the right mouse (context menu), I get strange behavior.
A group of them will open, MP3TAG will "flash", those will disappear and the next bunch will open, on through the end. When all the display fireworks end, only a small number of files are actually in MP3TAG.
Bunch is as few as 20 files, but usually more.
Opening the same files by pointing at the containing directory doesn't have the problem. Its also a lot faster.
Or I can drag all of the files in a group into an open copy of MP3TAG, and this works fine. Also faster.
Sometimes it works fine - in the sense that all of the files appear in MP3TAG even if slowly, but mostly not. On Win/XP, using release 2.44, Aug 14.