This occurs because the booking is distinct from the ticket.

A booking is a means of listing your name and details on a flight. Details stored in the booking apart from the flight details include seat selection, meal preference and frequent flyer number.

A ticket on the other hand is a contract for carriage. Without a valid ticket you will not be able to fly, check in or OLCI, whether or not you have a booking. Your refund will have invalidated the ticket.

BA will periodically scan their bookings for valid ticket numbers. If there is no valid ticket number the booking will be auto-cancelled. No doubt this will happen to your booking in the fullness of time.