Put a 1mm gasket between the "cuppi" and the body - it will ensure the pinion gets kicked out full stoke first then the contacts bridge.
I would also suggest to buff the solenoid piston (if removeable) and put the plunger in a cup of oil for about 5 or 6 hours, it will absorb the oil, once done then install it in the starter.
NEVER EVER EVER apply grease to the piston shaft - (as our cheetah electricians love to do) - it becomes tacky and makes the solenoid plunger stick.
If the plunger cannot be removed, then you are only stuck with installing a 1mm gasket or replacing the solenoid.
what is happening is that the starter starts to spin before the pinion gear (garaari) engages the flywheel. The thin gasket helps in such cases