Question Scriptable Renderer Feature And Stack Camera

Why the render feature on an overlay UI can affect the base UI?

I have a base camera for 3D objects, and an overlay camera for UI, so the UI camera stacks on 3D camera.

I am using a customized render pass feature to add blur effect on the cameras. It essentially blurs what the camera sees and makes a Render Texture for once. After the blur, I activate a UI image using this Render Texture so the 3D objects looks like blurred.




When I only add this feature on the 3D camera, only 3D objects are blurred. I thought that was because the renderer's feature only affect what the camera see and excluded stack camera.

I tried to only blur the UI by adding the blur feature to UI camera only, but UI and 3D objects are all blurred.



I moved the UI objects to left so it's more clear on the blur result: we can see the blurred image included UI AND 3D objects.
I don't understand how this happend, UI camera only sees the UI layer, why the render feature can affect other layers too?