[Reported as a bug] Extreme color banding in build but not in editor

[EDIT: Tried more things]
[EDIT: Reported as a bug]

I'm using the latest version of the post-processing stack via the package manager, in Unity 2019.1.5f1 (Mac OS version 10.14.5). In play mode in the editor, there is some minor color banding, but nothing beyond the usual. In a build, however, the banding is out of control. Any idea why this is happening?

I've tried:
- recreating the RenderTexture asset the camera uses
- changing the RenderTexture format (in the inspector and via code, to several different formats)
- recreating both cameras (the viewport camera, which outputs to a RenderTexture, and the UI camera, which does not)
- banging my forehead against the desk

 

[EDIT] Fixed this particular visual glitch. I have a dual-camera setup (one for UI, one for the viewport) and the UI camera's background color was changed to grey from black, probably during the last Unity editor upgrade. The grey only rendered through in build mode. Which is strange and bad, but not the source of the color banding. [/EDIT]

I thought I'd provide a brighter example and... it seems something strange is going on here.

 

To test things out, I created a new project and set up the post-processing stack from scratch. It seems like color grading is broken in Mac builds. Reporting as a bug.