[Solved]Multiple camera issue

Unity Engine
1

Hi.
I want to show HUD game view so I using multiple camera which to show UI.
These for UI camera “Clear Flags” seted “Depth only”.
I think the main game view showed below UIs, But the main camera view is no rendered.

I can normal display when the main camera “Viewport Rect” property change.But I don’t know why.

I Showed inspector for the camera’s property on the youtube link below.

If you set the W or H value of “Viewport rect” to 1 or more, it will be solved, but If there is a person who understands the cause, I would like to hear the cause.

Unity version is 2019.2.11f1.
Rendering pipeline is default.

2

This is solved.
This cause was “Directly to camera target” property on “post Process layer” component attached main camera.