Posterize with motion blur on.
[HitFilm Express] I have a moving variable speed static image (jpg) that I use to do a pan effect. I've turned on motion blur to make it look natural in the fast moving moments. In the composite shot everything looks fine (it sees and renders as it should). When I put it in the editor, the quality drops disastrously. There is posterization before and after the fast motion moment. This is how it appears on screen and this is how it is on the final render. When I turn off the motion blur in the composite shot, the video gets fixed.
I tried on different computers and the problem remains the same.
I tried with hardware rendering on and off. No difference.