Is automatic line breaks a feature of GFM?

[borekb]

According to GFM spec on hard line breaks, this Markdown:

one
two

should render as:

one two

This happens when GitHub previews Markdown files like README.md and similar but does not happen here in issues, for example, this is how it renders:

one
two

Is there an official spec on how GitHub treats user text written as comments to issues and pull requests? Is it fair to say that GH issues and comments are not GFM since they behave differently from the spec?

[Artoria2e5]

I swear it used to be documented as a part of GFM pre-CommonMark, but I really can't find such a mention now. Heck, I don't even know whether this library is the exact one they use in the backend...