Earlier quoted context omitted.
n.b. if anyone at Dropbox sees this or others want to reference it, here's the ticket: https://dropbox.zendesk.com/requests/3750448
That link doesn't work for me, says request not found. Can you clarify this a bit? I.e what does the Dropbox app do if you take a picture in portrait mode, and what does iOS do? Is the rotation info lost completely?
It's a lossless rotation - you can actually use something like jpegtran to rotate it back and the pixel checksum in ImageMagick's identify will match – but it completely breaks any system which uses checksums to detect duplicates, of course. I noticed when I started testing Google Photos and had a ton of duplicate images.