Using 2.46 on Win 7.
Steps to create bug:
Change to directory with sub-directories containing ~5,000 mp3 files with ID3v2.4 APEv2 tags.
Select all files. (Same problem exists if selecting only a file subset.)
Open 'Extended Tags' dialog.
Change TAG-A name to TAG-B.
TAG-B did not exist.
Can go back to original state using Undo Save Tag.
I can supply a before and after file demonstrating bug.