I've noticed if I start a comment with @ I get no auto-complete.
If I manually add a notification tag, does it get sent to the appropriate user?
I've tagged as bug, but if it's 'expected behaviour', that's fine.
They do work for me
This is a comment on this question:
The names that showed up were names from two users that had previously commented.
They work fine.
The system doesn't allow auto-completion when trying commenting on the post to ping the OP unless the OP has already commented. My bet is this is what you were trying to do, but without context it's impossible to say.
If this is not the case, the situation is irrepreducable for us.