When melee strikes to the head area register as "Brain" hits instead of head hits, there appears to be no damage applied.
If this is a bug, then well..it's a bug.
If this is intentional, then it would be best if it's impossible for melee strikes to register as "Brain" hits, since dealing zero damage when you hit someone in the brain with an axe is broken.
Logs of the seven strikes in the video. Only "Head" hits register. "Brain" hits do not: