Motion Blur creates dark artifacts on bottom of screen in editor

When motion blur is active and the view port is in motion there will be a dark artifact visible at the bottom of the screen. Normally this isn't very noticeable however the effect worsens with speed and increased motion blur. This is a regression from 4.11.2. (Compare screenshot from 4.11 and 4.12 using same settings.)

NOTE This doesn't happen in stand alone game seems to be a viewport issue.

1. Open attached project
2. PIE and run forward

Alternative Repro Steps

1. Open a blank project
2. Add a post process volume to the level (unbound)
3. Set the motion Blur amount to 1 and max to 100
4. Wiggle camera by right clicking and dragging in a circular pattern quickly

RESULT
A dark linear gradient like artifact appears at the bottom of the screen that grows in size relative to motion.

EXPECTED
No dark artifacts appear.

Dark artifacts at bottom edge of viewport

Motion Blur is broken/distorted