The back seat seatbelts in a 2004 Toyota Corolla Runx lock up and will only retract after being extended a set amount. The seat belt needs to be fully retracted for it to unlock. Both seatbelts do this so I believe this to be a design feature, but what is the purpose of this?
If I'm understanding your description correctly, you are describing a safety feature implemented to aid in the installation of child restraint devices. If a seatbelt is not "locked" then during normal driving the car-seat is capable of moving because the seatbeld is only partially secured allowing freedom of movement for comfort. While driving this creates slack that you and I correct for by using our muscles to counteract the forces which are trying to move us from the seated position. Since the child restraint is static it cannot move to readjust its position after every corner or brake application, we need to secure it so it adheres rigidly to the seat. There is a metal clip that may be used to bind the shoulder portion to the lap portion, thus binding the seatbelt, or you can achieve the same effect by pulling out the seatbelt fully, routing it through the proper channel of the child safety seat, as per the manufacturer's instructions, buckling, and allowing it to click back removing slack and securing the seat until it is released by fully retracting the belt after removal of the seat. Sorry for the marathon sentence. In this state the belt is prevented from loosening due to the slack created from small movement of the carseat while driving holding it tight at all times.