Booking.com has a setting called "Auto Replenish". This setting causes a room to open back up when a booking.com reservation is cancelled. It is totally independent of any availability sent by a property's channel manager and/or PMS.
When a booking.com booking is cancelled on the booking.com extranet by a guest, booking.com will automatically open the room and rate for the dates that the booking had been previously booked under, even if those rooms/rates are closed by the channel manager. This can cause unexpected availability which could potentially lead to an overbooking.
Changing the Setting:
This option is normally enabled by default but is agreed on in the commercial agreement between the property and Booking.com. If a property wants to change this, the property must ask their booking.com account manager.