This is a regression. Tested in //UE4/Release-4.27 CL18319896
Opaque material assigned to a Material Billboard attached to a Pawn renders differently against the Sky/Infinity when moving. Tested against different GIs with the same result.
The Material Billboard remains visible.
Where ever the Material Billboard overlaps the sky/infinity it becomes complete see through, and remains solid against everything else.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-161118 in the post.