I've looked into this further.
I didn't set Instant to be on, which suggests it maybe the default behaviour in settings.
Therefore you may not have changed it. If my previous suggestion
didn't fix your issue, then I think I have another answer for you.
In the Subscriptions
section of the FAQ, it states:
Instant Email Notification - sends an email as soon as a new post is added to the thread. This is not sent for every reply and will only be sent once until you revisit the forums.
Hope that clears things up.