It seems to be due to the addition of the bIsWaterPixel decision in Engine\Shaders\Private\VolumetricRenderTarget.usf.
ForwardRendering cannot access GBufferB.
Open NewMap.umap to see the problem.
Disabling the ForwardRendering setting improves the problem.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-174026 in the post.