Yeah. That's a common problem with forward rendering engines. In Godot, the maximum number of lights a single mesh surface can be effected by is 8. What's happening in the screenshots above is that Godot is trying its best to cull out the lights that aren't in view, and cannot fit in those 8 light spaces.
The only solution you can use is to break up the rooms into smaller parts. Each part still only has the 8 lights limit each, but you can support more lights as a whole that way. :)
