Many guns are very difficult to release the slide on with an empty magazine in place because you're fighting not only the friction of the slide stop against the slide and pressure of the recoil spring, but the pressure of the magazine spring as well. My guess would be that the reason you can release the slide more easily with the flush-fit magazine in place is because that magazine probably does not have as stiff a spring as the extended one does.
So long as you can release the slide easily with either a loaded magazine or no magazine at all, it's a non-issue. I really don't understand why it would be important to be able to release the slide with an empty magazine in place anyway, so I don't understand why you're all that worried about it. If you do have a problem with the gun, S&W has excellent customer service and will take care of whatever problem you may have. If you still think that there's something wrong with your pistol, you should stop filing on it and send it back to S&W.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar