I believe it has to do with the image info embedded into the file from the phone and the way the forum software interprets it.
When taking pictures with my Samsung Galaxy S8 for example. If shooting in landscape mode, I can hold the phone either way and the image will display properly on the screen whether the phone is held with the home button on the left or the right of the screen. The proper orientation for most any phone to take a landscape image is the home button on the right, though. If it is taken with the button oriented to the left, sure it will display correctly on the screen, but the image info embedded into it will cause a lot of other devices to show the image as upside down.
Same as in portrait mode - take photos with the home button facing down.