Well here’s what finally worked: setting the video mode to “Vertical HD 30fps” (and also “Vertical HD 60fps”) under “Non Broadcast” which was your first recommendation to user High Format in the other thread you linked.
Which would logically mean that if a user selects “automatic” under video mode, which is supposed to automatically detect dimensions, Shotcut is flipping the dimensions to make it 1920X1080 even with a 1080X1920 clip/still, and adding the black bars. Either because, as you said, the jpeg orientation flag is messed up, or Shotcut is being wacky. Either way, this could be why the forums are getting so many requests for help with getting rid of the black bars. 
I did look at the base image properties in GIMP, this was a photo originally taken in portrait mode with my android phone camera, and the orientation shows “right-top.” (There’s not really a “portrait” setting in GIMP to my knowledge.)