Curious bug. I'm still confused as to what this has to do with iCloud Photo Library though (or even with the Photos app). The bug as described isn't related to

Curious bug. I'm still confused as to what this has to do with iCloud Photo Library though (or even with the Photos app). The bug as described isn't related to iCloud at all, since your iMessages are stored in a local database and aren't persisted on Apple's servers (it sounds like the bug was the message thread was deleted from the local database, but not the message attachments, so the new View Photos feature was able to find those orphaned photos).