And even if you made it into a request, I doubt that anything would be changed, simply because of memory restrictions. You may have observed that the undo feature allows to undo several steps. If the embedded pictures would be stored to be undone as well, if would increase the memory requirements dramatically and reduce the number of treatable files just as badly.
So you are quite right: it is not a bug, it is just the way Mp3tg works for now.
AFAI have epxerienced it, the pictures are between some 10k and some 100k, whereas the total data of the rest of the tags hardly takes up much more than 4k.
So one picture is worth the tag data of a couple dozen files.
As requests have mostly asked for more files to be loaded and not less, the compromise is not to undo changes in respect to picture data.