It's most noticeable when the camera view is zoomed in and gameplay view is zoomed out, and when there are items on belt. The belt seems being switching between empty and loaded.

I was playing with mod Ultracube , which provides a camera view around where a cube is located. When player is near the cube, the player's normal game view will have some area overlapped with cube's camera view, and belts inside this overlapped area will flicker. I've attached a video showing this i...