I have an idea what it could be.
I am using my taskbar vertically on the right side of the screen. If I move the taskbar to the regular bottom location then the menu shows in the right place. But when I move it back to my vertical position, the menu appears horizontally where it would if the taskbar were on the bottom. So the menu has the y position of a bottom-most taskbar, but the x position of the right-side taskbar.