I had this problem on my Witness, on all mags. It was the mag catch, not the mags.
Lock the breech open and drop your mag and peer into the mag well with a good light and see if the shoulders of the mag catch are protruding into the mag well. This was the issue with my witness. I moved the shoulders back on either side of the catch a little with a dremel, problem gone.
Your magazine is being jolted by the recoil, and since these shoulders are touching the magazine, it's just enough to get the mag release to move enough to drop your magazine out.
Trust me on this one, it took me a while to figure out, but it was THE FIX for my Witness.
Hope this helps.