Ppq 45 issue.

Hi all, I've recently bought a ppq m2 45. I like the gun very much. I shot it about as well as a 9mm. Feels gr8 in my hand though, I put a rubber grip on due to it being a bit slick. So, I put roughly 280 rounds through it so far. Mostly 230 gr Winchester white box fmj and American eagle 230 grain fmj. About 100 rounds of my defense load, 230gr +p federal hst. And various other hollow points.
I had a bad habit of thumbing the slide release and locking the slide open. I have medium sized hands. Its never an issue with other guns that have smaller slide stops. I'm trying to retrain myself, not a big deal. However. I had the last round jump out of the mag landing on the ground unfired at least 4 times in all that shooting. I blamed it on me locking the slide back before last round chambered. Though I'd think it would stay in the mag anyway. The last time it happend I was pretty sure I didnt bump the slide. I've been keeping my thumb flagged up. I believe it's one mag not the other. But I'm out of shooting ammo. I will buy more Monday paying closer attention to which mag is which.

So, does anybody have any other ideas that may cause this? Thanks.