It is the Godot light manager turning them off.
Old OpenGL could only do 4, modern OpenGL can do 8.
The interesting part is that every object has it's own support for 8 lights. Meaning the more objects is in a scene the less noticeable the problem is.
Every object will calculate it's lights based on the ones nearest to it and the camera.
For the floor a grid like structure can be used to allow more lights:
Also large lights add to each other, obscuring the problem.