Render canvas onto a render texture alpha problem

I want to render a canvas onto a render texture but I'm having problems with the alpha values.

If I have a fully opaque panel containing a semi transparent panel, the area where the overlap occurs is semi transparent, even though the bottom panel is fully opaque.

Through my research, I've discovered it might be an issue with Unity's default shader using ColorMask RGB...

Render canvas onto a render texture alpha problem