The front UI element completely overwrites the alpha channel.

Hello, My game window has a transparent background, all transparent details “shine through” showing the desktop.
My problem is that the top element of the UI overwrites the alpha channel information without considering the bottom. In a normal game, this is invisible, but when I need to use the UI alpha channel, a problem appears.

Scene:

Back panel: color: white, alpha = 1
Front panel: color: red, alpha = 0.5
This is clearly seen when using a shader with GrabPass. Channels Output: (alpha, alpha, alpha, 1)
And since I use the alpha channel, instead:

I get this:

Example from the game:

(It is expected that the transparent red panel will not shine through because there is an opaque white panel behind it)

This problem persists on different versions (2018-2019) and in different color spaces (G / L). Forward rendering, orthographic camera. Canvas Render Mode: any.

Tell me please, is there a way to fix this?

 

Do I understand it correctly that this only happens with non-default materials?
If so, please post the code of the shader, if possible.

 

No, this problem is with the default material (standard UI / Default shader).
I found a solution to the problem in another section of the forum: for some reason, the standard UI shader uses Blend SrcAlpha OneMinusSrcAlpha, which leads to this result. To fix this, I had to change the standard shader:
Change blending mode to:
Blend One OneMinusSrcAlpha
Add line:
color.rgb * = color.a;
After that, I get the expected result (the alpha channel is not overwritten, but mixed correctly)