Hacker News defaults to using the canonical url set by the site's metadata where it exists. 99.9% of sites set this to the domain for better SEO, so any post from those sites will automatically be replaced by links to the domain itself.

Nowhere in the HN UI is the poster warned of this, because that would add complexity which would violate HN's principles of minimalism. You are simply expected to notice and re-edit the post within the edit window. Which no one does because they aren't made aware that they have any reason to.