This bug exist like forever. Apparently MP3Tag doesn't keep the imported or selected track/item when you select other window or the MP3Tag window get inactive. Then going back you still can type to the tags, but when you're about to save it, the buttons are greyed out, so you make sure you selected the real thing but when you reselect the item/track, it gets freakin cleared out.
Please fix this. This is some usability bug. This exist like forever on MP3Tag. I am annoyed on that workflow bug