After doing some testing with the dummies at Champion’s Gate, it appears that Perfection stacks are only proccing on the very first melee hit I do. I use Shadow Daggers (delivered via Umbral Blades) to proc Perfection and in accordance with the Exuberance node, I am receiving 2 stacks of Perfection on that first melee hit - but then nothing else happens.
No matter how much I continue applying Shadow Daggers (and thus using melee attacks), no more stacks of Perfection are applied. It appears that leaving the area and coming back simply resets the problem, allowing for those first 2 stacks of Perfection to proc but no more afterwards.