Turns out the issue is just as @8a54b27a (and others) described: https://mastodon.social/@ddlevine/111182827326542972
My“Daisy Mae Body” collection is a *person* collection, according to Photos, so it can’t be merged with a pet. @8a54b27a said he fixed this by deleting the “person” collection and then letting Photos do more analysis, resulting in a new “pet” collection which was then mergeable.
@a6ff0b2b Do you mind using the Feedback Assistant to report the issue?
@f89362b1 Now that I understand the issue, I’m not sure what the Feedback should say. Maybe just asking for an error message when trying to drag and drop a pet onto a person or vice versa?
@a6ff0b2b both issues: the dragging and the miscategorization of the pet body as human.
@f89362b1 FB13237070: Photos identifies my dog's body as a person
FB13237083: No error message when trying to merge a person and a pet in Photos