orthographic cam with 45 degree - z-index problem

Hi. I am creating a cool 2d game like enter the gungeon.

What I want to do is, this one. https://pbs.twimg.com/media/CDz7jp9WEAEMcpw.png:large

So I set my cam orthographic rotated with 45 degree
http://imgur.com/usu5dCy
that is,
http://imgur.com/a/eI6YH


And here is the problem.

In the scene, there're two object. green capsule and gun.
(having same sorting layer and order in layer)
http://imgur.com/AsWQA2Y
The position of green capsule object is [310, 276, -25]
and the position of gun is [295, 271, 0]

Sure, it looks like this in game view. green one is under the gun object. since the y position is bigger.
http://imgur.com/B2ZAT8a

but when I increase y position of the gun to 274,
http://imgur.com/cEuv2Yl

http://imgur.com/hkg8hlL
It renders weird. gun is now under the capsule.

Is this a unity3d bug?

 

+ I am using 2d toolkit which means sprite is rendered by Mesh renderer.