I assume so many people post a url then a period immediately after it to end the sentence, that HN strips off the period at the end of the url, assuming it's not part of the url and was instead meant to end the sentence. I can see that the period in your comment is not linkified, so HN considered it not part of the url. By doing this, HN is going against RFC 3986 which says that the period is an unreserved character and is valid in fragments (and paths) without being escaped.
You can work around HN's behavior by escaping the period as %2e