Signup query cache not invalidated when user added on multisite

[roborourke]

At the moment if a new user is added from a site on a multisite install direct from the admin (without skipping the confirmation email) they don't show up in the signups list.

Note that the first user added may appear in the list if the cache hasn't been primed yet so to reproduce you should try adding multiple new users.

The below code is a workaround I'm using for now:

add_action( 'after_signup_user', function () {
	wp_cache_set( 'last_changed', microtime(), 'signups' );
} );