Congrats on the CS9! Great little 9mm.
What you describe is normal. My CS9 does it and my 3913 does it all the time. Has from the time I bought it 23 years ago. And to date neither pistol has ever malfunctioned while firing. Never. Neither one.
Granted, I only have around 900 rounds through the little CS9 and in excess of 8000 through the 3913. But while initially troubling, the next round slipping forward has proven to be of no consequence.
In fact, with the 3913, I have come to expect the top round in the mag to come falling outof the magwell when I drop a loaded mag out to unload the gun.
I would NOT polish the stripper rail in the slide. It is functioning normally.
Should you experience a malfunction, especially a double feed, while firing, I would replace the follower and mag spring. A new recoil spring in a pistol with unknown round count is good insurance too.
Enjoy that fine little 9mm! Have you started hunting for a CS45 yet? You will be.
Regards 18DAI