No disrespect, gnorv, but folks often don't realize they have hit the slide stop.
Of course, you might have been shooting one-handed and so had no chance of hitting it, but if you were using a traditional 2 handed grip and thumbs indexed down the length of the gun, it can happen.
The only way I was able to convince another shooter that his gun wasn't malfunctioning, that his mags were not malfunctioning was to have him change from shooting 2 handed to 1 handed. When he shot 1 handed, he did not experience premature lockback. When he shot 2 handed, it would happen 1-3 times per mag. He claimed he wasn't touching the slide stop and believed he was, but apparently he was.
So you purchased 2 magazines from Wilson and had 2 problems, one of which you believe to not be caused by the magazine, but the ammo (stove pipe). So now you have 2 new mags and one malfunction you think is the mag. Given the sample size, no conclusions can be reliably drawn. However, if the mags needed to be broken in or were at fault, then I would expect that you should have had the problem repeated more times.