Empty query variables will not be included in a routed link. Take the following example:
<a href="<?php echo JRoute::_("&foo=bar&limitstart=");?>"> <?php echo JText::_('Start');?></a>
This link will actually exclude the empty
limitstart variable from the link. The resulting link will look similar to:
whereas you might have expected it to look like:
Another reason the variable may disappear unexpectedly is if the variable is manually
unset (removed) in the component router.