I just came across this article where everything I did it would just capture both monitors and even the alt + prt scn would not work. For me it was because I has Logi+ Options installed it overrode the windows settings so even though I had use print screen key to open screen capture on it would not work.
Going into Logi+ Options and and changing the print screen to just print screen fixed it for me.
Amazing.
Wonderful thinking around the box. Just shows how companies can do things but without communicating it change something entirely for users.
And I wonder why they felt the need to change.
To use their own software to harvest the image data?