I built a rifle a few months ago using an Anderson lower and PSA LPK.  I went to the range the other day and noticed that the bolt catch wasn't engaging when manually cycling the action. The bolt was catching on the back of the mag follower, so as soon as I released the mag, the bolt dropped.

I was thinking that there was something wrong with the rifle until I tried that magazine in my other, older rifle that's run perfectly for like 10 years and it did the same thing. A gen 1 PMAG in both rifles does not have this problem. It looks like the mag follower isn't always exerting enough force on the bolt catch to push it up far enough to actually catch the bolt.

Anyone else run into this? I ran 30 rounds through the rifle with both magazines, one round at a time, and the bolt always locks back after the last round so I'm not super concerned with it, but it annoys me enough that I'm probably not going to buy any more M3 PMAGs. Am I overreacting?

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I'm not much help but I do have a half dozen various pmags that my bolt will not hold open on after the last shot.  I mark them with a white dot of paint and move on.

Check the length of the nose of bolt catch. Some are slightly longer than others. The 9mm bolt catch has a much longer nose.