Mini-starter misadventure

When I installed the new drivetrain (318) in our 67 Barracuda, I replaced the gear reduction starter with a new mini-starter.

When the mini-starter functioned properly, it was nice. However, after starting the car 4-5 times, it intermittently would not engage to start the car. Instead, it would hammer the starter drive into the torque converter starter gear, kicking in and out, barely turning the torque converter. WTF?

I finally cleaned 1/4” to 1/2” of grease, grime and New Mexico red sand off the old starter and swapped the two. The old starter fired right up and has not done anything other than what it is supposed to do (so far).

Can’t see anything wrong with the new mini-starter. Can’t figure out what happened. Any ideas?