Go to components/com_k2/helpers/route.php

on line 111 you see this:

$link = 'index.php?option=com_k2&view=itemlist&task=tag&tag='.urlencode($tag);

 if ($item = K2HelperRoute::_findItem($needles)) {
 $link .= '&Itemid='.$item->id;
 }
 ;

change it to this:

$link = 'index.php?option=com_k2&view=itemlist&task=tag&tag='.urlencode($tag);

$Itemid = JRequest::getInt( 'Itemid');

$link .= '&Itemid='.$Itemid;