Pretty sure it orders them based on the fist occurrence of that sub menu name in the order the products are in the category.
Might be wrong but pretty sure mine are not in alphabetical order… and no they are not
Sort your Category Products so that there is at least 1 Product of a SubMenu at the top of the Product List. It’s assigned SubMenu will be shown first in the list of SubMenus, then the next Product belonging to a different SubMenu will dictate the second SubMenu to be displayed, and so on.