I had some tags named "ARTIST SORT" and when changing the name to "ARTISTSORT" unexpectedly resulted in the tags being deleted completely. I had multiple files selected.
EDIT: I suspect the root of the problem is that not all files had the "ARTIST SORT" tag.
EDIT 2: Tested with all files having the tag and same result. Changing the tag name on one file only works as expected.
Yes, I saw such bad behaviour too, but I am not sure if it has the quality of a bug or a design flaw.
When changing the tag-field name of a tag-field on multiple files at once using the dialog "Extended Tags" [Alt-T] Mp3tag displays the information what tag-field will set to and what tag-field will set to .
You can see in advance what will happen. Therefore the removing of tag-fields is not "unexpectedly".
I don't read it as that, because the "new" tag field appears as , plus if all values for the tag in the files have the same value it does function as expected.