Having 3 monitors and an XP-Pen Artist tablet (effectively a 4th monitor), I've had this annoying problem of working on one monitor, and child windows of a parent application opening on a different monitor to the parent. This has been really annoying, especially when using the pen on the tablet. Finally, I searched for a solution to this problem and found this workaround.
The first time a child window opens on the wrong monitor, use the key combination of [Windows Key]-[Shift]-[Right Arrow] repeatedly until the child window is on the correct monitor. Next time the parent application opens this child, it opens on the correct monitor.
I am hoping this is not the only solution as it does feel a bit of a workaround, so I would be interested to hear in the comments if anyone has a better solution.