Bit late but I just tested while upgrading 4.21-GameWorks. What you showed on your screenshot doesn’t happen for me, not in deferred nor in forward so I’d guess there’s some on the merge on your custom engine?
Wasn’t soon but it’s now included on 4.21.
I added FleX to my 4.21 branches, thanks for the trashbyte and karma4jake for the FleX fixes (also added small credits section to my GW branches readme’s):
(just UE 4.21.2 and Nvidia FleX)
(4.21.2 and Nvidia Blast, FleX, Flow, HairWorks, HBAO+, TXAA, VXGI 2)
(4.21.2 and Nvidia Blast, FleX, Flow, HairWorks, HBAO+, TXAA, VXGI 2, WaveWorks)