Got to be either a major draw or a major resistance somewhere.
<<< We have winner!
Major resistance = If you power direct to the starter motor with the motor still hooked up to the primary chain, the resistance is the engine resistance and cause a power drop this way... WATT are the odds?
Major draw = If we had the motor on the bench, took a current drop to the starter motor only, no resistance but the gear spinning in the air, watching the current draw down and then see if the motor by itself can begin to cook the starter.
Major ground = Where is the completed integrity of the ground from battery to ground? Engine to ground? Transmission to ground? NOLTT with a simple set of jumper cables to complete the ground. No engine pulled, no transmission pulled, no motor removed. Too simple is eliminate the ground first. No tools but cables needed.
Time is money = Which is faster? Ground the motor to the engine or bench the motor, find out NPF (no problem found). Replace motor back into bike. Reinstall primary cover/oil/gaskets. Does the customer eat the parts and labor now?
This is one variable is ground integrity vs. motor integrity. Just watching how the diagnostics moves: off of different approaches mentioned. Could be the motor, you never know? Could be a learning curve too... Out of who's pocket?