ASUS Splendid Display might be the issue.
Personally I love the program.. don't understand the hate about it.. love the orange filter to keep the dead white from straining my eyes. ;d
Although if you're duping the screen, I assume that's when Splendid would be applied?
Your initial connection might just need some default setting tweaks in the GPU menu..?
Never heard if this issue, will do a little poking, see how it works. ;x
❤