Tagger = Zortam MP3 Media Studio 5.85

Yeah, I was sure I had found the problem when I saw that the working file stores its image in the "other" field, not "front cover". However, copying or moving the image to "other" on test files has not worked.
But that does make me consider whether my testing has been sufficiently controlled. I'm going to try making modifications to copies of the working file and see what happens.