Testing and QA

That's a really interesting side effect that I don't think anyone anticipated :|

That's caused by two different behaviours:

In Moodle you subscribe to a forum, not to a particular discussion/thread. For example if you subscribe to the Gradebook forum on moodle.org you effectively subscribe to every discussion in that forum. The users in questions are subscribed to the forum even though they may not have posted in that particular thread.

Forum notifications are made to appear as if they've come from the person who submitted the forum post. In this case that is you. This has the unanticipated side effect that any auto-replies come to you.

This needs some further thought....

Me too. I am of opinion that it's not a good idea for people to put those automatic "out of office" notifications in their e-mail systems anyway.