You are missing the point here. Images are rendered from top to bottom. On mini 4, you don't dee this in portrait mode but in landscape mode when the top becomes left and bottom becomes right, you see left side rendering faster than the right side.
However on the mini 6, the display is rotated. What used to be on top in mini 4 is at the right in mini 6 and the bottom is at the left. Thats why you see it rendering from right to left.
And I can even see it on the iPhone XR now (in landscape) or is it just my eyes...