Back to the OP's question: I suspect(I'm not certain, just a guess) that there might be a "V" shaped spring in the trigger that makes the safety re engage. I also suspect that spring may have broken and caused the problem. Call S&W and send the gun back. I'm sure they will fix it.
Most new guns have some teething problems. From what I have seen the Shield has done pretty good so far and problems seem to be minor or at least correctable. Both of mine have gone "bang" every time I pull the trigger so far. That's what I wanted when I bought it. As to the white dots falling out. I don't care anyway. Mine haven't, but I prefer plain black sights. My first gun shot low and both right and left when I got it. I called the factory, checked on new sights and after 300+ rounds it now shoots in the bottom half of the bull, but in the BULL. I admit it was me. I had a very similiar problem when I first got my G19 too. Old 1911 shooters, especially those guns with light triggers, seem to believe it is the gun because it couldn't be them - right?
All in all, I like my Shields and hope they hold up to the S&W reputation. For now I'm happy, but if they don't, I'll move on to something else like I've done in the past. That's just the way it works.