I checked your website.
I see that it also need to add hidden fields for the parameters which are in the URL normally, otherwise, they are ignored.
So to do that, you need to also add such code as well after the previous line of my previous message:
<?php if(!empty($this->params) && $this->params->get('module') == 'mod_hikashop_filter' && ($this->params->get('force_redirect',0) || (empty($this->currentId) && (JRequest::getVar('option','')!='com_hikashop'|| !in_array(JRequest::getVar('ctrl','product'),array('product','category')) ||JRequest::getVar('task','listing')!='listing')))){
$menuClass = hikashop_get('class.menus');
$menuData = $menuClass->get($this->params->get('itemid',0));
$type = 'category';
if(@$menuData->hikashop_params['content_type']=='product'){
$type = 'product';
}
?>
<input type="hidden" name="option" value="com_hikashop">
<input type="hidden" name="ctrl" value="<?php echo $type; ?>">
<input type="hidden" name="task" value="listing">
<input type="hidden" name="Itemid" value="<?php echo $this->params->get('itemid',0); ?>">
<?php }else{
foreach($_GET as $k => $v){ ?>
<input type="hidden" name="<?php echo $k; ?>" value="<?php echo $v; ?>">
<?php
}
}