Why does my lighting create square boxes at sharp angles?

Took me a bit to understand what I'm looking at here. So this is a terrain mesh (viewed from above) with random thin mountains and red light in the middle, right?

Can you provide some sense of scale? What are the dimensions here? 100m? 10km? How close is the light to the ground plane?

Also, what are the coordinates of the center? Is this object near the world origin (0,0,0)?

This only happens with deferred lighting. Changing the camera's near and far clipping planes makes the problem worse, but doesn't eliminate it.

0,0 is the top left corner of the terrain.

Yes it is looking down at the terrain from the sky, but I get the same effect with a vertical plane and a camera looking sideways.

Scale of everything is the default: 1,1,1, except for the cube which I made into a small wall.

Moving the point light further from the terrain reduces the problem, but also means that I need to make everything taller to give off shadows.

The problem only happens at sharp angles so of course moving the light away from the plane would increase the range of non-sharp angles surrounding the light.

I don't know if it's related to z-fighting or not, but it seems like such a simple thing that I shouldn't need to waste time on.

I've tried changing the shadow bias and it has no effect.