Open, then open the level /Game/00_UDN_Issue/Reflection_Flickering.umap
Observe the light flickering on the edges of the screen when that edge is
SingleLayerWater
Go to the post-process volume, and, under Reflections->Lumen Reflections, change
Ray Lighting Mode to Surface Cache. Observe that the flickering no longer occurs.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-190508 in the post.