To reproduce:
1. Open attached project;
2. Open the "Sample" scene;
3. Toggle Post Processing on the Main Camera and observe the difference in the Game view;
4. In ForwardRenderer, find the NewCustomRenderPassFeature and change its event to "After Rendering Skybox" and toggle Post Processing again

Result: RendererFeature doesn't render anything when event is "After Rendering" and Post Processing is enabled; when event is "After Rendering Skybox, the renderer feature renders as expected.

Reproduced with: 2021.1.0a9 (URP 11.0.0)

Note:
If a user wants to have a renderer feature where the RenderPassEvent is controllable, they have to add code to determine if the feature should write to _CameraColorTexture or _AfterPostProcessingTexture.
_AfterPostProcessingTexture should be removed but meanwhile we should re-bind the RenderTargetIdentifier as _CameraColorTexture. With that users will not need to think about what the camera is rendering to, instead they just use the camera target.