Bit more googling suggests that Samsung devices handle rotated images differently, although I haven’t found a good source to confirm.
Seems like non-samsung devices might handle image orientation by simply switching X and Y, i.e. actually taking a landscape image as landscape.
Samsung devices apparently take all images with the same X and Y and then tag landscape ones with the relevant EXIF info.
So it seems like the issue here is actually a Discourse one as it really should be honouring EXIF orientation metadata.
@Roy_ILGM if you could post the same S8 image twice, once with untouched EXIF and one with the Image Orientation field set to blank then that might be proof enough to ask for it to be looked at.