Workaround fix: {waitlist_full} ... {/waitlist_full}

<?php

function my_em_event_output_show_condition($show, $condition, $full_match, $EM_Event){
	global $wpdb;
	switch ($condition) {

		case 'waitlist_full' : // waitlist is enabled and full
			$reserved                = $wpdb->get_var('SELECT SUM(booking_spaces) FROM '.EM_BOOKINGS_TABLE.' WHERE event_id='. absint($EM_Event->event_id) .' AND booking_status=6 GROUP BY event_id');
			$is_enabled              = get_option('dbem_waitlists');
			$is_fully_booked         = $reserved <= 0;
			$is_waitlist_available   = $EM_Event->get_bookings()->get_available_spaces();
			$show = $is_enabled && $is_fully_booked && $is_waitlist_available;
			if( $condition === 'waitlist_closed' ) $show = !$show; // opposite of closed
			if( $condition === 'waitlist_full' ) $show = $is_enabled && $is_fully_booked && !$is_waitlist_available; // only waitlist should be unavailable here
			break;

	}
	return $show;
}
add_filter('em_event_output_show_condition', 'my_em_event_output_show_condition', 100, 4);