So, looking at open issues in amethyst, it appears they have a really messy spam filtering system.
Apparently there's your blocked list, a "spammer" list that shows empty but actually has npubs in it that are blocked that you can't unblock unless following, a check box for warning you when your followers reported someone you encounter, and a check box to filter spam from strangers.
Somehow, there is hidden block list built into amethyst that you can't see, and the details of how accounts wind up there are really not well explained anywhere, but appear to be from other amethyst users reporting npubs, in some cases from any user and in others only your follows.