It seems the mob or trigger for the event has either changed, or is bugged. I suspect it's bugged, as I cannot open the door with the key, and picking the lock works. Handing Tyrian the key after the door is picked does nothing, and he gives the key right back saying he has no need for it.
There were some minor changes to Mistmoore that came along with the initial Depths of Darkhollow release, I have not tried to spawn the event until now, so I don't know if it was a problem previously. There is a post below that seems to indicate it has been happening before this, perhaps off and on, as it appears it's working for others. Anyone know what's going on?