1) Hmm. Split button's a special case vs a regular item/submenu. As Ivo explained after I discussed this issue with him, the reason the menu closes normally is that another item gets activated. For items with sub-menus, activation is tied to the opening of the submenu of another item. However in case of a split button, it's timer is different from the regular "Menu delay". So the first expanded menu will close after the split button delay time passes and it opens. You could always hover over the right part of the button to open it with the Menu Delay.
2) A harmless minor bug.
Thanks for reporting.