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.