It is related to hitboxes going out of sync with a characters model when they get freezed. Excellent breakdown of the bug by Mitchel here.
So if the “latest” nerfs to Mei wasn’t enough. Mei’s projectiles are now literally disappearing / doing 0 damage. Imagen spending all your ammo on freezing a Genji, having just enough ammo left to get in a headshot, and then the projectile just disappears in thin air. Fan-flippin-tastic! Link to video below showcasing said scenario.