Material and lighting bug?

I have noticed when I increase the brightness of textures in a material to create a glow, the lighting gets weird when it isn’t a circle.


The expectation is that it would illuminate in a circle from the visible space of the texture. In the example of my moon here, the glow should be covering the dark side of the moon.

I tried using a crescent moon texture with alpha space for the dark side of the moon. This weird light cut still happens.

This weird cut in the glow feels like a bug. Is this normal? Is there a way around it?