When an object move, it becomes brighter step by step, not continuous?

I made a projector in my scene, it’s in a room with mostly static lights and two stationary lights. When i played in third-person mode, it becomes brighter step by step when it going down, like when going a certain distance it suddenly became brighter. Like the picture shows.

By the way the object is in a blueprint actor.

Any thought on why this happen? I want it to become brighter continuously and smoothly, to make it looks real.