If (e.g.) you hit the first target with a greatsword swing or warscythe reap, but miss the next two, you only get one +8% bonus from Fast Adaptation, not two … surely not intended …
Em…I guess it is intended.
In version before, maybe before v1.005, things are like what you have speculated, any single attack in a aoe is roll seperately and will add a stack or consume all stack of fast adaption/head hunter . But after then only head hunter keeps the same, but fast adaption will not stack more than one stack in a aoe attack.
As it is changed intentionally, I guess iths working as intendes.