And the others that all use the same image? Does it make sense to you that all of them have been changed?
Here’s a screenshot of settings which as you can see (and as I’ve told you), hasn’t got any backgrounds set in it. I can’t give you admin access because we have customer data on it, but this should settle any doubts.
It just appeared. As it did more than once for your other customer according to their post.

Please just fix the bug.