HEllo,

You should move the mouse straight to the side and after that, you move around within the field of the menu to keep the hover event effective .
When you move the mouse away from the field, there is no hover any more, and sub-menu disappears because you have set showing the sub-menu on hover.
This is the nature of the hover event. If you do not like it, switch to the click event. This is not changeable.

Best Regards