This problem can be found in version 1.0.4, because the function submit_post is used with an empty username.

Take a look at the source code:
submit_post('edit', $this->data['post_subject'], $this->data['post_username'], …

The var 'post_username' is only set, when the username is Anonymous, otherwise it is empty. It seems the function submit_post converts empty names to guest an writes this to the topics table.

solution:
$username = ($this->data['post_username'] == 'Anonymous') ? $this->data['post_username'] : $this->data['username'];
submit_post('edit', $this->data['post_subject'], $username, $this->data['topic_type'], $this->poll, $post_data, true, true);