Bought a shield 9 last week. beautiful gun, very comfortable.
Anyways, I cleaned it out and oiled up the gun like the instructions said and took it out to the range today and around round 40 my slide locks back. I counted the rounds while shooting and realized "I should still have one more in the mag". So I try to release the slide lock and realize its actually stuck PAST the slide lock. It took me a good knock into the back of the slide to get it back into battery. Is this just a breaking in issue?
I checked the rails and didn't see any abnormalities. I did see a small divot (about a half millimeter) on the inside of the rail of the slide's frame but idk if its even big enough for anything to catch it. its about a half inch past the slide lock's catcher (idk the specific name for it).
So after I got home and started cleaning the gun I did some tests. I pulled the slide back as far as I could and slowly released the slide to see if I could get it stuck like it did at the range. Yup. But I just can't seem to figure out watch's causing the jam.

I've only found 2 other older threads on this issue and i didnt want to revive the old threads.
Any ideas or tips?

Remove slide and post of upper and lower assemblies

Yes. I had the same problem except my slide locked back on the 4th round in the mag. I basically did the same thing you did, I cleaned the pistol right out of the box and took it to the range. 4th round it locked back and I had to hold the pistol upside down and tap the top of the barrel to get it to back into battery and therefore let the slide to move forward. I tried it again and the 2nd round did the same thing. I used factory ammo. My friend with me tried it and it did the same thing to him. I returned the pistol to my local dealer which in turn sent it back to Smith. I got it back in about a week and a half. I tried it out this past Saturday and it works fine. They didn't say in the paperwork what they did to it. But, it works. You and I are not the only ones. Send it back.WG