Put your starter motor on your work bench, and take the inspection plate off. Look at the commutator segments and brushes. If the segments look good (no black burn marks, taper or egg-shape, go on to the brushes. If the brushes look bad, replace them.
Your Bendix gear is supposed to disengage anytime power is interrupted or when the flex plate rotates faster than the starter motor.
If all looks good, your Bendix gear is probably going south. Buy and install a new one or trade-in your starter motor for a remanufactured one. - Dave