I can confirm that, yes, the ball bearings DO fall out if you keep undoing that screw (it's easier than you think!)
It's happened to me once during a match, and a handful of times in training, and now it's just become part of my drill to make sure the screw is done up again fully after I undo it to release it. Hasn't happened since.
I can also add it is ALMOST impossible to get the BBs back in again, and a real pain in the a*se.
I told MEC about the problem, and asked what I could do to prevent it, and they sent me a new front part of the sling. As I eventually found the missing BB (it was in the turn up of my jeans...) I now always carry a spare in my kit bag
In short, great sling, just be careful when undoing it and ALWAYS do it up tight immediately after you release it from the handstop.