I popped a photo in one of my threads yesterday, taken on the iPad (running the latest iteration of the operating system), to see if the forum software did indeed spot the EXIF and rotated the image correctly.
It didn't.
Now, normally I take images and add them to a post as I need them, from the image picker. I choose "Existing Image" and navigate to my Photos collections, choosing from there. In the Case of the Inverted Image, I actually took the image from the point of choosing to upload an image. In other words, rather than picking from Photos, I simply used the image right away. I don't know if that had a bearing on things.