bbPress.org

Usernames can have a space in them because there is also a user_nicename set. This is all lowercase and any spaces are replaced with underscores. If you look at the link to your member page here you’ll see it ending in ‘james-blacker’.

I presume you’re wanting a link to the profile page when you click on a username.

To do that you need to something along the lines of

<a href="<?php user_profile_link(get_post_author_id()); ?>"><?php post_author(); ?></a>