Well… that’s not good 
I dont have 4.21 GW built atm but you could try reverting commit from 4.21-Gameworks using git:
after that HBAO+ renders directly on top of color instead of AO RT. Downside is that commit was used to have AO have lesser effect in direct light so that will not happen anymore (you’ll see the AO effect on sunlight then unlike with unreals own AO) + HBAO+ will not work on forward shading enabled anymore.
If it’s still leaking after reverting that commit, then it’s on Nvidia’s VXGI branch itself.