The following code displays the external links symbol:
<p> A </p><p class="plainlinks"> [https://github.com/Nikerabbit GitHub] </p>
because the resulting HTML is:
<p> A </p><p class="plainlinks"> </p><p><a rel="nofollow" class="external text" href="https://github.com/Nikerabbit">GitHub</a> </p> <p></p>
This looks completely unexpected to me and seems only to happen when whitespace is place in this way.