Permanent FPS Drop After turn 360 around stand still

I have the following trouble: I’m standing still in the game and I have stable 55 fps (on the testing spot). After a few seconds, I look around 360 degrees, and looks back the same testing spot, and my FPS drops down to 40 fps, and never gets back to 55. I’m using stationary dynamic lights. It very strange for me and I cant understand why this is happening. Can somebody explain why this could happen?

It looks like this is solved in 4.21, but I still don’t know what was the reason. Probably I never will.