Assassin cobra bug (reproducable)

Hallo everyone,
I am playing in offline mode and noticed a strange bug (which does not exist in online play).
I am running an assassin with mosaic build and all the skills work fine on their own, but when I have my charges up I get a strange bug:
cobra on its own, no matter if lvl 1,2 or 3 leeches life and mana perfectly, health orb does not drop below 80%, but as soon as I get charges of fists of fire life leech goes down to such a small amount that I have to keep triggering health potions and even the instant potions to stay alive.
Anything know about this issue?
As said prior this only occurs in offline/single player mode. I am running the same mosaic build in the ladder season and there everything works fine.

FoF gradually converts more and more of your physical dmg to fire and once the skill is lvl 33, or above, it converts all physical dmg to fire. You need physical dmg for cobra strike to function, so in the end it leeches less and less as FoF gets stronger and once your FoF is lvl 33+ cobra strike has no way to leech resources at all. Even life tap won’t work after lvl 33.