iOS 15 "Shared with You" disappearing Photos

Say I receive a Message with a photo in it -- there's an option in Messages to save the photo and I tap it. As expected, now I see that my photo is saved in Photos.

Now suppose I delete the message that contained the photo. You'd expect the photo to stay in Photos since I saved it, right? Well, at least at first, it does stay there. But once I plug in my iPhone 13 to charge, after a while, the Photo disappears! It's like iOS decided that since I deleted the Message that sent me the photo, clearly I don't want the photo any more either (even though I specifically saved it!)

I've found an (annoying) workaround. After saving the photo from Messages, save it *again.* A dialog comes up telling me I've already saved that photo; I choose "Save Duplicate." Now, in photos, I have two copies of the image, and if I look at the info for them, the first one is "Shared with You" and the second is "Saved from Messages." The "Shared with You" one will eventually disappear after I plug in, but the "Saved from Messages" one will stay forever.

Anyone else seeing this? Seems like a bug to me -- a "shared with you" item should go away if the underlying message is deleted, but NOT if it was pinned/saved, right?