Question How to perform a full screen blit in Single Pass Instanced rendering in XR include transparent objec

As title, I want to made an effect in VR headset based on URP.
I found this topic and it works.

https://docs.unity.cn/Packages/com....eatures/how-to-fullscreen-blit-in-xr-spi.html

But it is weired because transparent object was missing,
I changed the example
_CameraOpaqueTexture to _MainTex
still failed, how to fix that? thanks.

This is the scene include transparent object...


After color blit transparent object was disappear...