Completely not true.
Oh so I imagined this exact scenario when my extractor ripped the rim off a cartridge and the next round was halfway out of the mag smashed against the stuck round in the chamber.
That I had to physically reach in through the ejection port push the round back into the magazine to get the mag to drop free.
I must have imagined this.
How exactly do you get a mag to drop free when the round is still under the mag feed lips but not out of the mag entirely in an AR15.
Magic?