I’ve recently done my environment in UE 5.3 with Lumen and faced those shadow/lighting artifacts. I tried raising Final Gather Quality setting in Post Process Volume as it helped lots of people that had the same issue but it doesn’t help for me.
do you have any water with rendertarget or something similar?
I had the similar issue , it was driving me crazy,
At the end I had to delete any Render target from any water/landscape and use a different approach
, the crazy flicker was gone after