Email templates - user_watching_first_post - not available

You recently added another email template that was missing (custom_invite_forum_mailer). Thanks!

Could you also add this one that I just noticed is also missing - user_watching_first_post? You would think that it would use the user_posted template, but it doesn’t.

Furthermore, when looking at mails sent with the user_posted email type, it isn’t using our subject line correctly. Is there more logic to it than meets the eye?
We have the subject as: [%{site_name}] %{subject_prefix}%{username} wrote a post in '%{topic_title}'
However, a new topic (without any posts), just generates [%{site_name}] %{subject_prefix}

I did some digging, and I found that user_watching_first_post actually uses the user_posted template.
So why is the standard subject line being used?