Closed dates are still available!

The property is listed, by the room or for the whole unit, on Airbnb & VRBO as well as Booking.com, and all calendars are linked. A guest just booked a room in the property via Airbnb for several days, but the Booking listing still shows the whole thing's available. I've marked it as closed in Booking's calendar, I've read through the related help articles, and I can't figure out how to block it from the calendar for this date.

See screenshot, which marks the listing as both closed and "1 left to sell". What does that even mean? I CANNOT have this double-booked, especially if Booking makes it hard to resolve! How do I ensure that if a room is rented that it's blocked from Booking's calendar?