Is this phenomenon an aspect of the Glock or the retention holsters many cops use? I have heard that using the trigger finger to push the release on the holster as you draw has a tendency to put the finger inside the trigger guard as the gun leaves the holster. Having used a type 2 retention holster (Blackhawk) I can see how this would be the case.
Somewhere there is a rule about not leaving a round in the chamber, but there is plenty of debate around here regarding that rule.
I have a Galco holster for my Glock. The trigger and guard are actually recessed into the holster. One grabs pistol by grip and jerks it out. Then place finger on trigger guard. Took a while to master this and adjust holster properly.