I check all my files with mp3val befor etagging them, to take out the most common file errors.
Now, I sometimes come across files that have tags across the board and if I change one entry (in, say, author), and then save, the entry gets deleted entirely. Anything I add into that field afterwards gets deleted. Same goes for the image: If already present, and I try to add another or change it, it vanishes. This, I cannot add again in WinExplorer, so the file will remain bare afterwards.
The entry used to be visible in iTunes and Windoes explorer, but after changeing in in mp3tag, the filed is then empty in all other programs. I then have to manually add some text into the then-empty fields using WinExplorer / File Properties / Details. Then, iTunes at least can find the entry again, though the line remains empty in mp3tag indefinitely. This is not specific to this version, but has occurred acros many different versions.
I have seen this with mp3 as well as MPEG-4 Audio File (m4a) and MPEG-4 Audio File protected (m4b) files, though most prevalently with the latter. The files are not protected as per file details, and are not in hhidden or read only mode. It also doesn't matter if I run mp3tag as admin or not.
edit: 2 sample files that didn't work:
type m4b, one coded with Nero AAC codec / 188.8.131.52 (57 kBit/sec, 22kHz, mono), the other with Lavf56.40.101 (62 kBit/sec, 22kHz, mono)
Seems to be something specific to the files that mp3tag reads incorrectly?