According to the documentation:

You can not depend on any particular contents of the RenderTexture you get from GetTemporary function. It might be garbage, or it might be cleared to some color, depending on the platform.

You'll need to  clear the render target explicitly, after getting it.

That is correct: RenderTextures are not necessarily cleared after being released or reacquired.

According to the documentation:

You can not depend on any particular contents of the RenderTexture you get from GetTemporary function. It might be garbage, or it might be cleared to some color, depending on the platform.

If you want the RenderTexture to be cleared, then you'll need to clear it yourself after acquiring it.