Well, broke down and ordered three 9mm metal followers from Al. Today at the range with my K9, the slide locks open with one round left in the magazine. The first time it happened, I thought I must have hit the slide lock or something, paid extra attention next mag, and same thing, locked open with one round remaining. Now, here's the kicker. With my carry guns, I always practice one handed, strong side, one handed weak side. When I did the weak side (left) hand, worked like a champ, no issues with premature slide lock. I think I might be pulling my little finger under the mag and putting extra up force on it, not sure. Anyone else observe this strange behavior? Never happened with the factory followers.
I did not do the left hand thing until late in the session, will have to give more thought to the problem and see if I can repeat next week.