As noticed by jverdaguer on moodle.org and reproduced on https://sandbox.moodledemo.net.

Steps to reproduce:

1. Enable the guest login button in the site administration.
2. Create a standard forum in a course with guest access and self enrolment enabled.
3. Add a new discussion topic.
4. Log in as a guest and go to the forum.

Expected behaviour:

The user sees a 'Add a new discussion topic' button, which if they click on it prompts them to create an account and enrol in the course in order to be able to post.

When viewing an existing discussion, the user sees a reply link, which if they click on it prompts them to create an account and enrol in the course in order to be able to post.

Actual behaviour:

The user doesn't see a 'Add a new discussion topic' button nor any reply links, so doesn't have any idea that they need to create an account or enrol in order to post.