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
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that is,

And here is the problem.

In the scene, there’re two object. green capsule and gun.
(having same sorting layer and order in layer)
Imgur
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.
Imgur

but when I increase y position of the gun to 274,
Imgur

Imgur
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.