Same problem and causing a lot of issues at the moment! My properties are only in airbnb and booking.com. Unfortunately, I have noticed that very late!!! I got a reservation via Airbnb on December 2018 for the date of December 2019, so almost year ago... Then I got another reservation via booking.com April 2019 for the date of December 2019. There is 4 months between two new reservations and the calendar did not sync. When I called booking.com, this was the answer " You lost your connection in some point and did not refresh your calendar" What an earth! When I asked that how I could know if the booking.com lost the connection and the answer was "You should check the calendar settings time by time".
The extra information I got is that the booking.com calendar sync itself every 8 hours.
This is unacceptable from such a big company. The technical problems can occur, that I understand but we should get the notification to handle our cases manually. At least we should have opportunity to avoid over bookings.