The passive “siphon” on the acolyte skill transplant does not seem to be able trigger from pools of blood you teleported on top of, in case of short range uses of transplant or like the pool left by “reign of blood”.
It is very easy to test, due to transplant costing health. The healing does not trigger instantly nor by walking out of the pool of blood and back into it. Even worse, if the bugged pool from “reign of blood” touches the pool at the departure point, that one stops being able to heal too.
If this is by design, the tooltip of the siphon passive needs to be changed, since it makes no mention of such a restriction and even uses plural for pools of blood in the detailed explanation, despite the pool from “reign of blood” (the only way to get another pool of blood) not working.