When melee strikes to the head area register as "Brain" hits instead of head hits, there appears to be no damage applied. I believe this is because the brain health coefficient vs. a melee attack is currently set to '0'.
If this is a bug, then well..it's a bug.
If this is intentional, then the problem appears to be that unlike projectiles, melee cannot do damage to multiple body parts (in this case, the head and the brain) with a single attack. If you could, the player would be guaranteed to at least get the head damage (multiplied by whatever modifiers are involved). However, the game chooses one of the other. If it chooses "brain", then the player gets screwed.
Logs of the seven strikes in the video. Only "Head" hits register. "Brain" hits do not: