Thanks @Junkie. I used the same mags with this gun that I do with my other P365 (with no issues on that one) and have swapped this slide to the other frame and experienced same failure to return to battery. Issue seems to be the slide.Mark your magazines and see if there is a particular mag that causes the problem. To me breaking in a gun is very counter to all I have learned about firearms.
@CovertCarry yes, I have a couple of grayguns grip modules that I like to use (which are stock Sig other than some laser engraving for grip). I've swapped out parts, even used the romeo zero slide on my SAS FCU (which cycled 100%).Have you had the FCU out of the gun at all? Might be worth looking at and getting a good look at rails and any spring loaded parts.