I found that if permalinks are on and the post-slug contains the word ‘WordPress’, the comments section would not appear on the post. I have submitted an issue to the WordPress team.
If you notice, I have changed the word ‘WordPress’ in each of the slugs to ‘wp’ and they work fine. An old post with WordPress in the slug can be found here. Notice – no comments section.
Obscure issues like this is why I don’t mind installing beta software in my production environment. Chances are, if you don’t run it for real, you won’t find this stuff.