Sorry, I'm still not able to reproduce this.
I have a similar setup with my main monitor being 2560x1140 and my secondary monitor being 3440x1440. When I expand or go full screen, it maximizes correctly on both monitors. I've also tested with my primary monitor resolution set to 1920x1080.
I don't know what could be causing this, but until we are able to reproduce it, it will be hard to fix.
Which graphics card do you have installed?
