Vibrating knife cutting pattern is not closed.

There more causes:
1 Servo motor delay
2 Knife shape 
you can try to solve it by the following:
1 set the knife rise up and go down delay whick means the knife go down and delay for some time and the start cutting. when finished a closed shape cutting, a rise up delay is inserted to wait the path to meet the start point
2 Tune the servo motor to be a higher responsibility to decrease the motion delay of motor