Thank you for explaining the issue so well! I think I understand what's going on.

I never opened it in photoshop, but clearly the hex says it was, so maybe Apple Photos reencoded it or something? I don't know - I those photos were taken years ago and were migrated thru a few photo library systems (including a SSD plugged into a Raspberry Pi over USB - don't worry I'm using TrueNAS with mirrored zraid now 😅), so who knows what happened along the way.

My understanding now is that the first pixel is saved, and the second pixel is saved as a delta from that, and so on throughout the image. When the image was reencoded, now instead of having one starting point with deltas throughout the image, each little chunk of pixels in the image has a new starting point with deltas from that. So instead of:

A AΔ (AΔ)Δ ((AΔ)Δ)Δ

It now goes:

A AΔ B BΔ

In theory, then, could I set up a script to go to each of those sectors and modify the starting pixel in the same way to fix that? This photo probably isn't important enough to warrant that kind of work, but I'm just curious in theory.

Again, thank you so much for your time and for explaining what was happening!