This is more a bug report, than a feature request. CorelDraw does not suffer this problem, only Photo-Paint.
If Corel Photo-Paint is minimized and you add a monitor, then when restore Photo-Paint, at least on my PC, it always opens to the size of the full screen. It is NOT maximized. It's still a free-floating window, but it's size is set to the size of the entire monitor. I have to manually resize it back down every time this happens. On a 43" 4K monitor, it's very rare that I want anything to run the size of the entire screen.
I don't think it matters how many monitors are in use, but I happen to have 3 total on this PC.