I'm using Pro Builder Basic and found an issue when creating corners. I've tried searching the forums for a solution but without much success.
Steps to reproduce:
Starting with a basic ProBuilder cube
I extruded one face out to the side
And then extruded the face upward again, which causes the 'gap' to appear.
From what I can tell, there isn't any actual gap in the mesh. I've played around with the lighting settings some with no success either.
The light that I'm using is the default directional light that Unity makes for a new scene.
Operating System: Windows 10 Home
Unity Version: 5.4.1f1 Personal
ProBuilder Basic Version: 2.6.1f0