This happens when Airbnb reads "Default prices and availability" given in Airbnb when the Booking Automation calendar is "closed out" with no availability.

Please update the settings below in order to make sure that Airbnb does not fall back to "Default prices and availability" when there is no availability in Booking Automation.

Settings > Channel Manager > AirBnB.com XML > Specific Content > Pricing > Dates with no price = Make unavailable