The strangest light building shadow artifact Ive seen, someone please tell me what's going on here

Since 4.20 or 4.21 I have been getting these super weird shadow patterns into my light build. I’ve seen it in different Unreal projects of mine.
It looks like some sort of texture that is getting inserted into the light bake…? Does anyone have a clue what is going on here?

This has to be a bug in the engine, I’ll file a bug report

They appear like crop circles…