just on the off chance here, cos i haven’t actually gone here myself yet:
i’ve been moving stuff around in my own forum, and to avoid people getting notifications of old stuff i have temporarily disabled ALL outgoing email.
but when i created a new post just now, i did receive a regular notifying email of that post.
when i checked other users, i realised it was just me, as admin, no one else had received that outgoing mail.
so maybe the same thing is happening here: that no matter what settings you use for length of excerpt, the owner/admin gets the full mail.
just a thought.