That lighting effect is called “Auto Exposure” Its there to simulate realistic lighting (though ive always found it to be pretty annoying) There is some way to disable it in the editor, but i dont remember how. Instead what you can do, is Add a Post Process Volume, make it the size of the room, i THINK the section its under is called “global illumination” and then you can set the minimum and maximum values for auto exposure to 1 and 1. That way its technically still on, but will completely get rid of that lighting effect