I’ve been looking into this problem for a while. I think it’s innate to path tracing of Lumen. For a consistent surface, PT just choose some rays to trace and blend them together. Since Lumen is also sort of screen space rendering, the sample amount is good when it’s in screen, and bad when it’s out of screen. The problem goes extreme where there are shadow and indirect light. So the reflection of out of screen part is provided by Lumen hardware ray tracing, I guess if you increase this sample resolution, it will look better, although it’s a performance trade off. For me, I just drag in a PP volume, and set each variable related to Lumen to a really high number, let’s check out before and after.
before
after
What matters the most is the fisrt value “Lumen Scene Lighting Quality”, which controls the overall sample resolution of Lumen.