How exactly did you wire your gear reduction starter?
Vendors often supply a little jumper from the battery terminal to the solenoid terminal and suggest you wire it similar to a stocker by simply reconnecting the big cable back onto the starter.
Problem with that is that the still spinning starter becomes a generator when you let off the key, which can keep the solenoid engaged, which keeps the pinion engaged with the flywheel.
I'm trying to imagine how fast the armature of a PMGR starter would be spinning engaged to a flywheel turning at cam break in speed.