Bright brim / edge on specular material

Hello!

I’m trying to create a puddle.

For my tests, I have created different puddles.
I have painted the alpha channel in different ways to see the results.

If I paint a rectangle with fully painted alpha channel (with no transparency at all), I expect the puddle to be a perfect rectangle as well.

However, Unity creates a bright brim / edge around this puddle.
I don’t see why.

What might cause this bright brim, and how do I get rid of it?

It’s in the bug tracker now.