It is sliding forward. It releases where it supposed to at the end of the lips however it springs upward pretty hard as soon as it's released because of the mag plate pushing up on the bullet head.

I double checked and the bullet is definitely reaching the chamber before it is released from the mag lips.

I can recreate this with my finger just by sliding a bullet in with the head up slightly above horizontal, as soon as the angle on the neck gets to the chamber it catches.