That's not the part in question. Strength of Blood permitting instant leech is intended, and the 3.15.3 patch notes made that clear.
The part that seems like an exploit is instant leech applying to energy shield when life is not full (but unreserved life is full). It's difficult to rationalize why Immortal Ambition would work this way.
The part that seems like an exploit is instant leech applying to energy shield when life is not full (but unreserved life is full).
Yeah, this part is a straight-up bug, and will be squashed at some point, in some way. The rest of it is just a good-quality breakdown of some lesser-known/understood mechanics, as expected from sergeantminor.