The nobles aren't respawning every 6 minutes. I've timed it to be around 13mn instead. Isabella's respawn is also incorrect, although I didn't time it properly.
This was previously patched back in march but I guess it got broken at some point as I can't seem to find any recent patch notes about their respawn so I'm assuming the change to their respawn timer wasn't intended.
Relevant thread.
It's kind of minor but it does make exping in HHK far less appealing. =/