Booking.com is from 1.8.2021. included a new general auto-replenishment feature.
Please read this article to the end to avoid overbooking!
Whenever a cancellation occurs via Booking.com, the room is automatically returned for sale to give the property another chance to resell it. Although you have already closed the calendar in the meantime, b.com will open the date automatically. This means that potential overbooking may occur although MyRent sends them the exact occupancy. More information can be found at:
In order to avoid potential overbooking, it is necessary to deactivate the “auto-replenishment on closed rooms” function and you should personally contact Booking.com technical service or through the extranet.