For how much time did you keep the lock ? :)
The radar don't drop your target instantly when it goes out of the search cone, it will keep the target and update its position with the data it had before it exited the search cone, and try to require it when it goes inside the search cone again.
But after some time the lock will drop.
The radar display mode your are using is projection, vertical sides of it represent the extremities of the search cone, that is 60°.