In case of sensorless, I think that the problem is the circle limitation. When the motor generate a high BEMF that is quite equal to the VBUS available (decreased by the death time, ...), the circle limitation change the angle. For better understand ...

The problem is inside the PID of the speed, the integral part. There isn't the anti-windup in case the PID of current can't set the output of the PID of speed. A possible case is low bus voltage.