I can only speak about the C++ API, but as to this, this behaviour is normal. IIUC, the BP3 motor API has PD controllers which approximate the target and then keep active continuously to track the target further on, and even correct the motor position if the motor axle is moved manually in between.
To stop the PD regulation, just add a “stop” command after the target has been reached (either by an active brake or just coasting then).
HTH!