Force the EE event calendar to always use the internal links for events even if an external url has been set.

tw_ee_filter_tooltip_event_permalink.php

<?php // Do not include the opening PHP tag if you already have one.

add_filter('FHEE__EE_Calendar__tooltip_event_permalink', 'tw_ee_filter_tooltip_event_permalink', 10, 3);
add_filter('FHEE__EE_Datetime_In_Calendar__to_array_for_json__url', 'tw_ee_filter_tooltip_event_permalink', 10, 3);
function tw_ee_filter_tooltip_event_permalink($permalink, $event, $datetime){
    // Force the calendar to alway use the event permalink on the site, even if an external URL has been set.
    return get_permalink($event->ID());
}