Light shaft bloom produces heavy flickering when offscreen

See attached video.

Happens always in editor, will also happen in standalone game but only if viewport rect has not the full window size.

The issue was not present in 4.22.

Update 1/22/20: 

A large gray radial glow flickering on and off as the camera moves in particular directions, only present when there is a directional light with light shaft bloom enabled.

1. Create a blank project in 4.24.1.
2. Open an empty scene.
3.  Add a directional light actor.
4. Rotate the directional light so its close to parallel with the XY ground plane.
5. Enable Light Shaft Bloom on the light.
6. Move the camera around.

 
Results: As you move the camera, you should see the gray flickering artifact appear.
 
Expected: In an empty scene, the light shaft bloom shouldn't show at all, because the bloom shouldn't do anything to a pure black background.