This question is an exact duplicate of:
When using the mobile theme (e.g. mobile device), the add comment link disappears is a comment is added and deleted.
Steps to repro:
- Open a post (I'm using Firefox)
- Add a comment
- Delete the added comment
Now the particular post for which the comment was added & deleted, doesn't show the add comment link. Reloading the post makes the link appear again, however.
And it's gone:
EDIT: This appeared to be fixed a few days back but seems to be broken again.