I'm going to guess that pict-rs (the image uploader behind Lemmy) is stripping the EXIF data that controls orientation, and that when you're "editing" the pictures before uploading, your editing software is just modifying the EXIF orientation data without re-saving the raw image in a new orientation (as it should). I checked into the pict-rs source code a bit and it appears that's what's happening via a sledgehammer exiftool -all= call, but I don't have any familiarity with the pict-rs codebase so I could be mistaken on where this is getting called. It should probably be preserving the Orientation tag with this exiftool call, as well as maybe some color profiles etc. In the short term, I would say upload your images to a different image service or use image editing software that forces a re-save of the image itself instead of just the EXIF data.

Photo orientation is stored in two places. The place that is a tag gets stripped with all location data on upload.

Source:I am not a developer, so this is repeating what I read elsewhere. Correct me if I am wrong.