The event is built to trigger as soon as you want into the mansion. It starts with her saying "So... This is it, huh?" Phoebe will also bother you about sealing her Pact if you haven't already.
After the event finishes, a switch is turned on that allows you to get into the swamps.
Interesting indeed. I'm wondering if you aren't at a different part of the game.
I'm happy to help troubleshoot if you're stuck, though. What's the Main Story task you have in the Task List, what what's the most recent incomplete objective?
