OK got a temporary fix.
if you look in listing-filter-form.php @ line 8 you will see:
<form class="listing_search" name="listing_search" action="<?php echo home_url();?>" method="get" >
I changed this to:
<form class="listing_search" name="listing_search" action="<?php echo home_url(), '/';?>" method="get" >
and that fixed this issue in this case