Same thing on my new Taurus. For me this was a manufacturing defect, I have never dropped the hammer without the slide present. It was a very square ridge where you could clearly tell that machining step was either not finished or performed incorrectly. I also found a gigantic burr in the slide stop hole, and a few other little things that don't impact the function of the gun but lead me to believe they don't have the best QC in the world.
Manual cycling of the slide felt very "rough" and sometimes when dropping the slide release it wouldn't go fully into battery. Under live fire the gun cycled and fed just fine, despite this problem.
I stoned off the high spot, cleaned and lubed and now she is running like a real champ
I got rid of the factory mags after my first range trip. One worked fine, the other one would not reliably lock the slide back on empty and would not drop free. I'm not sending them back to get fixed and don't trust either of them even though the one works fine. Power Mags solved that problem.
I still think this is a great gun for the money, but would not recommend it to anyone who isn't willing to deal with this sort of stuff themselves or send it back to Taurus and wait for them to fix it under warranty.