I added a sub menu to a Moodle custom menu doing:
$parent = $branch->add(...to create the parent and then:
$parent->add(...to add items to it.
The hover works ok on larger screens but once collapsed on mobile the sub menus wont expand on click.
On mobile the sub menu displays but clicking it will not reveal the items below it.
I notice this does not work in clean when you add a "custom menu" either.
Has anyone got nested custom menus working on mobile in other themes?