Some sites drastically change their interview processes for Phase 2, especially since the Phase 2 interview process happens on a much shorter timeline than Phase 1 so it can be difficult or impossible to free up interviewers (ie if a site isn't able to cancel already scheduled patient appointments during the interview window).
If the open house is optional, the site likely feels comfortable ranking and matching with the invited candidates without ever meeting them. If the open house is required, ranks might be adjusted based on things like interpersonal fit.
Some sites also have brief interviews (such as a single interview with the training director or a faculty member) so I imagine it's also possible to fit that into a more extended open house setup (first hour = intro & Q&A panel, then individual breakouts for interviews).
And yes, you can absolutely clarify with the training director on if there will also be formal interviews. Good luck!
