Acho que é porque você pode adicionar coisas como .rss, então se você terminar uma frase entre parênteses com um link seguido de ponto final, o Discourse acha que tudo faz parte da 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.)