Lack of post author notification of a comment in a thread under their post

[trichoplax]

Raised in Discord:

In the post https://codegolf.codidact.com/posts/296063 there is a comment thread labeled Output format. User wizzwizz4 left me a comment in that thread but I didn’t receive an inbox notification for their comment. Why not?

Note that the lack of notification is for another user joining the thread. There had already been comments from the user who initiated the thread and the post author. The post author confirms that they are a follower of the thread (it says "unfollow" at the top), and that they received notifications for the comments from the thread author, just not for the new commenter.

In addition to the comment that lead to no notification being from a user new to the thread, it was also the first comment in the thread in which the post author was @ pinged. I don't know if this is relevant.

Note that the following screenshot is from my user, so says "follow" at the top, unlike for the post author, where it says "unfollow" at the top. The new commenter that triggered no notification is wizzwizz4:

[trichoplax]

I've been able to reproduce this in my local development environment:

1. Go to an existing thread under a post.
2. Add a comment that @ mentions the post author.
3. Sign out and sign in as the post author.
4. See no notification.

A comment including an @ mention of anyone other than the post author is notified to all relevant users correctly. Only an @ mention of the post author prevents the post author being notified. When the post author's notification is suppressed, all other users are still notified correctly. I haven't yet identified the cause.