Bug Post process layer tears apart the screen when I change viewport rect

I'm changing my camera viewport rect at runtime to make letter box but, the screen is torn apart like the following tweet: https://twitter.com/Juicefoozle/status/1091805214443798528?s=20&t=Iwv60iQblul3n3acaWq4bw

Is there any known bug that occurs the problem like that in post-process?

Or is there any way to go around?

Any help is welcome