Problems using "Motion blur"

Hi!
I´ve got a strange problem under Linux and Mac when I set active "Motion blur" in my main camera and using a lower resolution than the native: the post-processed image "jumps" to the top of the screen. This doesn't happen if I use the max native resolution or if I have a camera rect like X=0,Y=0,W=1,H=1.

This issue happens with the "Noise Effect" too.

Does anybody know what could be happening?

Thank you!!

Modify:
I added this images to clarify:

When blur is inactive:



when blur gets activated:

 

I'd recommend you get amplify motion which is on sale at the moment, it's in orders of magnitude better than the unity motion blur.

 

Thank you, hippocoder! The problem is not only "Blur" but all the other post processing effects. None of them works correctly. :-(

 

It happens with all postprocesing effects, even if I only have this on onRenderImage

void onRenderImage (RenderTexture source, RenderTexture destination){
Graphics.Blit(source, destination);
}

It seems like the RenderTexture ignores the cameras rect.

Do you know if there is a way to modify the "source" offset or something like that?

Thanks!