can only guess- perhaps the starter gear is not engaging the flywheel fully?
As they mark the ring/pinion setup in a rear end you could check what it looks like
with grease. Open the inspection cover and look (two bolts)

Are the teeth breaking of the flywheel only at a certain depth on the tooth?
Two different ways to improperly engage the gear, one from bendix not engaging to the forward horizontally. The deepness of engagement in the vertical direction is determined by the starter design and any shims installed.

In any event, if the starter is messing with the flywheel, you should not wait around before purchasing a new/rebuilt starter made for the application, as changing a flywheel soon will be a REAL pain and expense