The featherfoot skill Blood Sucking only acquires targets when the skill is first cast.
If you begin casting Blood Sucking and the target moves away from you, even outside of your screen, you will continue to damage them and drain life from them for the entire duration of the spell.
If you begin casting Blood Sucking and a new target moves into the area of effect, this new enemy will not take damage or recover your health.
I doubt this is intentional, but if it is then disregard this post.