It looks like the DistantScreenTrace(...) in LumenScreenTracing.ush reports a false positive hit in the dark regions.
Using screen trace visualization reports the distance in these regions as 65535.
1. Open test project from Licensee: https://udn.unrealengine.com/sfc/servlet.shepherd/document/download/0694z00000Wy9GjAAJ?operationContext=S1
2. Observe dark artifact on the reflective rectangle when nothing else is present in screen space
3. Turn off “r.Lumen.Reflections.DistantScreenTraces” and observe that the problem is no longer occurring
4. Alternatively, change the cvar: “r.Lumen.Reflections.DistantScreenTraces.MaxTraceDistance” to a lower value like 10000 and observe that the problem is no longer occurring.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-200202 in the post.