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The problem when belt is going out of the pulley is caused by a slight tilt of shaft either on the side of the motor or ball bearing. That is the reason why is there the washer and why the printed pulley on the stepper motor have flat sides. If possible use washers on both sides of the ball bearing so the belt won't slide down to the rough surface of the plastic part, but will run on flat surface of the iron washer.
Also try to set lower microstepping. You should be fine with 1/8 concerning the precision and the motor will gain a bit of torque.