Strange dirty borders in postprocessing after upgrading to 4.9

We just upgraded to 4.9, and we’re noticing the a grimy dirt effect all around the edge of the window now:

This definitely has something to do with the post-processing effects. We do have a post-processing volume in our scene, but I’m not seeing anything that would cause this.

Why did this suddenly start happening, and what do we need to do to turn this off and get the grime off of the screen?

Oddly, if I delete the PostProcess volume and replace it with a new one and set the properties to be EXACTLY THE SAME, the problem goes away.

I hate to call this a “fix” because there’s clearly some other underlying bug in UE4 allowing this to occur … so even though I can “fix” it on my end, Epic should be aware of this.

This looks similar to what happened to me – the post-process build failed. In my case, it was because I was using a scenetexture:color in a post-process, which used to be OK, but now needs to use postprocessinput0.