Issues with saving a RenderTexture

Hello,

I have one code path that works, but another that fails. I am trying to figure out why.

Works
1) Project has a RenderTexture asset
2) Scene has a camera using RenderTexture asset as targetTexture
3) Camera is actively rendering to the RenderTexture asset
4) Call this code
-- Create a new RenderTexure and assign it to camera.targetTexure
-- camera.Render
-- RenerTexture.active = the new RenderTexture
-- Create a new Texture2D
-- texture2D.ReadPixels
-- texture2D.Apply
-- write texture2D bytes to disk as png file
5) Png file looks proper

Fails
1) Project has a RenderTexture asset
2) Scene has a camera using RenderTexture asset as targetTexture
3) Camera is actively rendering to the RenderTexture asset
4) Call this code
-- RenderTexture.active = camera.targetTexture
-- Create a new Texture2D
-- texture2D.ReadPixels
-- texture2D.Apply
-- write texture2D bytes to disk as png file
5) Png file is solid grey

So, the primary difference is that I am trying to not do another camera.Render and create a new, 2nd RenderTexture.

I am trying to just save off the contents of the existing camera.targetTexture.

I have verified that the camera.targetTexure is correctly updating, because it is also rendering in a UI RawImage.

Any help is appreciated.
Thank you
Les