RST back-ticked anonymous redirection not recognized

[rpuntaie]

pandoc 2.1.3

a.txt

`x`__

__ `xy`_

.. _`xy`: http://xy.org 

does not work:

>pandoc -f rst a.txt
[WARNING] Reference not found for '%60xy%60' at line 1 column 6
<p><a href="">x</a></p>

works:

> rst2html.py a.txt
...
<p><a class="reference external" href="http://xy.org">x</a></p>
</div>
</body>
</html>