I don't think you got this is right.
cameras and telephones preserve the vertical horizontal setting of a picture ion the Exif ( exit data) and , normally, when you tilt the camera the accelerometer changes this, but you may very well have changed this in your setting and not be aware of that. The forum system reads the Exif data to publish.
I have uploaded 3 files, from Internet , as an attachment and from my camera they all show vertical.
So you either have set this up wrongly in one or more hardware (or software) settings OR your accelerometer is stuck therefore everything shows horizontal
Hm, I'm not really sure what's going on with the Exif data... I don't think it's anything with my smartphone (iPhone X) accelerometer since it displays the correct orientation in my photo library on the phone. However, when I view the photos on the phone through Windows 10 File Explorer, the thumbnail shows the images rotated, but opening them up shows the correct orientation.
Then when I move the pictures from my phone into another folder on my computer, File Explorer shows both the thumbnail and image in the correct orientation - but when they are uploaded to a website they are rotated incorrect again. So I'm not sure at what stage the mix up is occurring or what setting I could even change - I looked at both iPhone and Windows settings but couldn't find anything. Windows 10 supposedly reads the Exif data for displaying photos as well.
If I open a photo with the Photos App and save it as a copy (even without editing anything), then it saves correctly and is able to upload correctly, so that could be a temp workaround.