I keep the keltec mags fully loaded in the stock. Have never had rounds eject, nor have I had the magazines drop on their own with heavy 75gr ammo. I store the rifle in my safe bolt open, loaded 10 rd C Defense Products mag in the well and have never had an issue. The C-Pro mags drop free with some "flick" upon depressing the button, and it gets easier the more I use them.
Consider checking out the pins on the stock that fit into the depressions on the mags. Mine have a very tactile and audible click when I put them in and take considerable force to remove.