How to ensure add_submenu_page works properly

I was trying to add a submenu item for my Event Calendar Newsletter plugin using the following syntax:

function ecn_pro_license_menu() {
add_submenu_page( 
    'eventcalendarnewsletter', 
    __( 'Event Calendar Newsletter Pro', 'event-calendar-newsletter-pro' ), 
    __( 'Pro Settings', 'event-calendar-newsletter' ), 
    'add_users', 
    'ecn-pro', 
    'ecn_pro_license_page' 
);
}
add_action( 'admin_menu', 'ecn_pro_license_menu' );

Where eventcalendarnewsletter is the slug of the parent menu item, ecn-pro the slug for the submenu item, and ecn_pro_license_page the function to render the settings page.

While the submenu item appeared, it was not working as the url was wrong:

It should be /wp-admin/admin.php?page=ecn-pro, not /wp-admin/ecn-pro.  Double checked the syntax, looked at other examples, and all checked out…

The issue was that the add_submenu_page() function was firing before the add_menu_page(), due to the priority of the admin_menu action call.  Adding a later priority sorted out the issue (in the add_action call):

function ecn_pro_license_menu() {
add_submenu_page( 
    'eventcalendarnewsletter', 
    __( 'Event Calendar Newsletter Pro', 'event-calendar-newsletter-pro' ), 
    __( 'Pro Settings', 'event-calendar-newsletter' ), 
    'add_users', 
    'ecn-pro', 
    'ecn_pro_license_page' 
);
}
add_action( 'admin_menu', 'ecn_pro_license_menu', 99 );