Basically, if you use one Heavy Attack then the first light attack immediately following does 60 base damage plus bleed. It should only be doing 25 damage plus bleed, so it’s incredibly unbalanced. It also seems to effect all subsequent attacks when alternating from heavy attacks to light attacks.
Further testing seems to indicate that alternating from light to heavy also increases damage.
It appears that current melee damage is added to the next attack as long as you alternate between heavy and light attacks or vice versa.