This is definitely an issue.
While sticky, the default mobile menu can’t be very large, or this will happen.
Looking into it, there’s not very many solutions with the way it’s currently set up.
The solution for now if your mobile menu is too long while sticky is to use the Slideout menu on mobile – it allows for longer mobile menus regardless if it’s sticky or not.
I’m on the lookout for ways to make the default mobile behavior work with a sticky menu as well.