Scriptable Render Pass Doesn't Render After Post Processing

I’m Trying to make a posterization/pixelation screen effect that includes the post processing within the effect (i.e. the vignette shouldn’t render over the pixels, it should be pixelated as well). I’ve been able to successfully prototype the pixelation and posterization effect and get it to work as a scriptable render pass, but only if that pass happens within RenderPassEvent.BeforeRenderingPostProcessing. If I change the renderPassEvent to RenderPassEvent.AfterRenderingPostProcessing the effect simply doesn’t render at all.

The thing is though, the effect is there, as you can see in the Frame Debugger:

But it just goes away during the final blit:

If I had to guess, the problem is that the final blit is referencing a texture that my custom render pass isn’t writing to, but I don’t know what I should write to (or how) to get it to work.

Thanks in advance for any information.