@Charlotte870
If a guest wants to terminate a long term stay, then something must be done with the reservation.
Not just keave it as it is.
Options:
1. The guest cancels the reservation and get refunded according cancellation policy. For long term stays it means next 30 days from day of cancellation are not refunded. But the calender is made free for new bookings
2. The guest or the host uses the "change" option to shorten the reservation, with same result or by making other deal (only possible if host is making the change). The calender is made free.
But you (or the guest) did nothing !
You contacting Airbnb and they just cut off the reservation. Which is rather weird, but that's how they sometimes act. And it can/will not be reversed.
This all could be avoided by simply take control yourself about reservations, as mentioned above..