If you use Tempo I’d recommend switching to Tempus, a fork that is actively maintained. Tempo has been unmaintained for a few months now.

This is what I suspected: if OP has a folder structure with folders for each letter AND all artists from the same letter share the same image, then quite probably Jellyfin is taking the parent folder for the letter as artist, looking for the closest artist name it can find and using that image for all the content of the folder.