Sometimes images are presented rotated through metadata, rather than actually rotating the image. Check the metadata on the original to see if it includes this. Then open an issue on lemmy's github project, if there isn't one already.
Or maybe it's just its way of telling you that you should be taking pictures like that in landscape orientation.
I do not know but I think the problem arises because the EXIF data gets stripped when you post so the orientation information is lost? If the EXIF orientation differs from the actual orientation, it will be rotated after uploading.
I thought it was because of the orientation of the pic, but I posted another one later with the same orientation, and it was correct. I wonder what causes this.
I also thought it was somehow my picture, even though I knew it was in portrait.
I was trying to post while lying on my bed so I though it was because my phone was in landscape mode somehow, which it can't because I don't enable auto rotation, so I gave up.