Creo que es porque puedes añadir cosas como .rss, así que si terminas una frase entre paréntesis con un enlace y luego un punto, Discourse piensa que todo forma parte de la URL.
[Link: https://meta.discourse.org/t/new-to-discourse-start-here/1.]:
[Link: https://meta.discourse.org/t/new-to-discourse-start-here/1.]
[Link: https://meta.discourse.org/t/new-to-discourse-start-here/1]:
[Link: New to Discourse? Start here!]
[Link: https://meta.discourse.org/t/new-to-discourse-start-here/1?u=jonathan5.]:
[Link: New to Discourse? Start here!]
(Link: https://meta.discourse.org/t/new-to-discourse-start-here/1.):
(Link: https://meta.discourse.org/t/new-to-discourse-start-here/1.)