Reading LightAttenuationTexture Produces Unexpected Results With Forward Rendering

The expected behavior occurs in 4.19; unexpected behavior occurs in 4.20 and 4.21. Marking this as a regression.

Per User:

Reading the LightAttenuationTexture in a material receives interference from other textures under forward rendering. When viewing the output of the material preview window unrelated textures (usually masks used in other materials) will appear on the preview mesh depending on where the camera is looking. In addition to this, the LAT output within the viewport appears to be affected by another texture, as though it is receiving the lighting data, but has also added the other texture on top of it.

1. Create new Default project in 4.19
2. Enable Forward Shading in Project Settings, Restart when prompted
3. Create a new material as shown in attached screenshot
4. Drag some Basic Cube static meshes into level and apply the new material to them
5. Rotating light source results in the "Expected" gif.
6. Now open project as a copy in 4.20. 
7. Rotating light source results in the "Result" gif.