Magazine problem

I just got a .40 p99 a few weeks ago with 2 factory blue magazines.  I’ve only shot it twice and I put about 350 rounds through it.  The first time I shot it one of the magazines broke open from the bottom while I was shooting and the spring popped out.  I exchanged it for a new magazine at Sportsmans Warehouse.  I only got to shoot about 50 rounds today but I noticed both magazines have cracks on the plastic at the bottom. One is loose at the bottom and it looks like they going to break from pressure.  Does this sound like a manufactures defect or could I be doing something wrong?  

Also I don’t know if this is normal but it won’t let me load a magazine with ten rounds if the slide is closed.  There can only be 9 or less rounds in the clip if it’s going to be loaded with the slide closed.