My bet is that you were very weak when racking the slide from prone position, the round came loose from the extractor but didn't make it out of the gun, and then the slide came forward and just rammed the round in to the chamber backwards. I am right handed and would have had to tilt the gun to the left to grasp the slide with the left hand so the port would be up, not down. Then just bump the ejector enough to loosen the round, have it flip 180 but lie in the breech face area, and get loaded backwards.
I'd put a snap cap or two in a magazine and try to duplicate it on the floor prone. My bet is its all related to being prone, worst position for being able to rack the slide with any speed.
Was the magazine latched in or free when the jam occurred? If it was latched in, then maybe you pulled the slide back far enough to bump the ejector and free the round that never fully chambered but not far enough to catch the next round in the magazine. Again, seems to me to be consistent with prone position weak attempt to rack the slide.
Any idea why the gun jammed in the first place? Was it the first round in the string or after a reload or mid magazine?
Joe