temp-fix-bug-97829.php

<?php
function ecp_fix_tag_order( $sql, $query ) {
	// Only modify the main query, if it is a tag archive query
	if ( ! $query->is_main_query() || ! $query->is_tag() ) {
		return $sql;
	}

	$has_event_start_date = (bool) strpos( $sql, 'AS EventStartDate' );
	$orders_by_post_date  = (bool) strpos( $sql, 'ORDER BY post_date DESC' );

	if ( $has_event_start_date && $orders_by_post_date ) {
		$sql = str_replace( 'LIMIT', 'ORDER BY EventStartDate DESC LIMIT', $sql );
	}

	return $sql;
}

add_filter( 'posts_request', 'ecp_fix_tag_order', 20, 2 );