Turns out the issue is just as nostr:npub13f2ty735een5d5ujkdg8w53eg3pdrt7q3cjtwmjnul3gdhucy6yqr0z5p9 (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. nostr:npub13f2ty735een5d5ujkdg8w53eg3pdrt7q3cjtwmjnul3gdhucy6yqr0z5p9 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.