The mirror takes about 10 seconds or drives above 10 mph in order to return to its normal position. So when you try to parallel park, the mirror doesn't keep moving.
One advice for winter, DEACTIVATE the Park Assist by pressing the "LEFT MIRROR SELECTOR BUTTON" or if you have right hand drive, then do the opposite. This will prevent any damage to the mirror or motor if the mirror is frozen with ice/snow.
1999 E430 Sport