Strange lighting/shadows in Unreal Engine game (shadow suddenly gets darker or lighter)

Question

As the title suggests. I have made a level for Unreal Engine. It uses RectLights in the ceiling without any directional lighting. There is a Lightmass Importance Volume and Post Processing.

I show GIF animation to show the problem, but similar stuff happens all over the maps I have made.

Is this a problem from the Light, the Level Geometry or something else?

Thanks