Set type to Artist cover image

I dont now if it is a bug or not, but sometimes when i set an image as artist cover, it does not save:
Add the image:
imagem

change the type:
imagem
Save or restart the mp3tag, it stays at front cover:
imagem

What kind of file does show this behaviour?
AFAIK not all file types support all cover types but only one like front.
see e.g. here:

This file
folder

i use an web source to put the artist in mp3: Audible.com#Authors images.src (2.5 KB)

i save it as front cover first and then change it manually to artist.

is that an mp4 file? then the cover type can only be set to front or back

ha it is, thank you.