I can, but D and I put our heads together to troubleshoot this afternoon, and the problem might be on this end.
Specifically, it looks to be be something introduced into Preview since the upgrade to Big Sur that causes Preview to halfway turn a JPG into a PNG in certain instances. (The Adjust Size and Save windows won’t tell you that, and the extension still comes up as .jpg, but the file size triples and the PPI resolution drops to 72 no matter what it’s supposed to be.) We’re pretty sure that that’s where the problem is cropping up, we just haven’t figured out why.
If it makes more sense to close the bug report, then please do. Or, I can email the images that are glitched, as well as the originals—which have all of the original metadata intact, so far as I can see. I’m open to either option, so let’s go with whichever one works for you. Thanks for looking into it, @tiwane!