There’s an with sprites in 4.15. You should use 4.14.3 for now to solve your 
I’ve spent days trying to figure out , literally, even reviewing the VXGI builds evolution since june 2016, only to fix one.
That’s how i figured a tessellation got broken last june.
But for specific, i’m afraid it requires a deeper understanding of VXGI integration in Unreal engine that i can’t afford to get into right now, it’s all in @ hands ^^.
I would assume there’s a shader for vertices to fly all over the place like that. You can use the Matine test sample or even the ShooterGame sample to highlight the.