Good question. When I had to make decisions about the starter motor, this did worry me. Yes, the starter motor remain engaged after the motor has started. The life time of the starter motor bushes is therefore in question. However, this allows the starter motor to be used as a dynamo to charge the battery.
Just be careful that the engine does not over rev the starter motor….there is a risk of bursting the windings on the armature….I know…I have done it on a car! The starter got jammed with the flywheel gear…
Thanks for that information.