I have problem with fullscreen on secondary monitor.
In my test is the second monitor on left (-1280)

This work well in 4.8.6 & 5.3.2

When I used show() on secondary monitor, its show on defined point. But when I use switch to fullscreen window jump to primary.

What happend :
1) show() = secondary monitor
2) switch by showFullScreen() = jump to primary
3) swicth by showNormal() = jump to secondary
4) switch by showFullScreen() = stay on secondary

Test project attached.