When post-processing enabled on camera, Unity ignores custom projection matrix?

I'm using the following code to set oblique near clipping plane on a camera (not the main camera):

camera.ResetProjectionMatrix();
camera.projectionMatrix = camera.CalculateObliqueMatrix(clipPlaneCameraSpace);

This works fine as long as no post processing is applied to the camera. However, when I add a PostProcessLayer to the camera, suddenly objects in front of my oblique near plane are rendered. Using the Frame Debugger, I can see that in this case the projection matrix (well, unityMatrix_VP) that is being sent to the shaders is that is as if I never set my custom projectionMatrix. That is, Unity is calling ResetProjectionMatrix internally. In fact, it seems to be calling it in camera.Render() (my camera is disabled and I render it manually). Known issue? Intentional?

Unity 2018.3.6f1

[edit: it seems to be happening sometime between OnPreCull and OnPreRender]

 

Aha, looks like I answered my own question... I didn't realize the source code for the post processing stuff was available:

Code (CSharp):

1.  
2. void OnPreCull()
3.         {
4.             // Unused in scriptable render pipelines
5.             if (RuntimeUtilities.scriptableRenderPipelineActive)
6.                 return;
7.  
8.             if (m_Camera == null || m_CurrentContext == null)
9.                 InitLegacy();
10.  
11.             // Resets the projection matrix from previous frame in case TAA was enabled.
12.             // We also need to force reset the non-jittered projection matrix here as it's not done
13.             // when ResetProjectionMatrix() is called and will break transparent rendering if TAA
14.             // is switched off and the FOV or any other camera property changes.
15. #if UNITY_2018_2_OR_NEWER
16.             if (!m_Camera.usePhysicalProperties)
17. #endif
18.                 m_Camera.ResetProjectionMatrix();
19.             m_Camera.nonJitteredProjectionMatrix = m_Camera.projectionMatrix;

 

Answering, because I ended up here using Google to solve my problem:
setup an oblique frustum (custom cam matrix) + using Post Process Stack V2

Solution: use the (new) physical camera - "Lens shift" (see Camera inspector) instead of a custom matrix in code..

 

i don't care who my hero was. but from this moment on, you are my hero. mua~

 

so how to fix this issue?
I'm using the following code to mirror screen (main camera):
void OnPreCull()
{
camera.ResetWorldToCameraMatrix();
camera.ResetProjectionMatrix();
Vector3 scale = new Vector3(flipHorizontal ? -1 : 1, 1, 1);
camera.projectionMatrix = camera.projectionMatrix * Matrix4x4.Scale(scale);
}
This works fine as long as no post processing is applied to the camera too.

 

This seems to be fixed in the latest 3.2.2 post-processing package.