When you assign a lemming a blocker when they are close to another, they won't get turned into a blocker. This makes sense; you can't have overlapping blockers.
However, when you make a lemming a blocker too close to another one, it will make them a blocker after they have reached a sufficient distance. I do not think this should happen; if you assign a blocker to close to another, you should have to manually reassign it, as making it happen several frames after assignment is not consistent with how the player should work.