This forum post explaines the bug perfectly.
Im also including another thread found here, listing similar bug reports describing the same issue.
Aimed shots at frozen targets head don’t register as headshots due to hitboxes changing location when frozen. This has been in the game since atleast June.
I discourage any Mei player to freeze targets if you find yourself in a 1v1. With her current ammo, thats a clear path towards suicide. Time to up your aimgame folks.
Anyways, stay safe <3
Ps. Yes I know you hate Mei and she is the devil, but this is just messed up.