Bring the holster with you to Glock. The holster may be pushing the slide out of battery slightly, just enough to cause the firing pin not to drop on the live round. Then with the slide out of battery the holster may also be somehow pulling the trigger back, that's the only way i can figure this can happen?
You got yourself an X-file there for sure. I don't see any other way how this is possible.
If the gun is defective and the trigger is getting pulled somehow and not firing when holstered you are the most blessed man on the planet right now. Don't push your luck anymore.
Please update us after you goto Glock.
You should see me buy scratch off tickets