Good question.
Originally the orientiation was set to Rotate 180. The photo appeared the right way up in Digikam.
I set it to Normal and nothing changed in Digikam.
I then set back to Rotate 180 and the photo rotated by 180 degrees in Digikam.
I then set it back to Normal and the photo rotated to the right way up in Digikam.
I uploaded this photo, and Grav displayed it fine.
I had tried other, online, EXIF editors before this. None of which made any difference to the uploaded image. Digikam meant I could quickly change and test out different orientations.
But I have no idea why this particularly combination of settings works.