I'm kinda surpised there aren't any discussions on the matter, makes me feel a bit lonely... Or most probably I'm missing them.
By the way, I've been affected by this bug for something like 2 weeks and it's driving me crazy: I'm getting all the known debuff given by the passive (heal debuff, shields debuff, etc) but it seems like the 50% damage mitigation is just not working, as I see some crazy damages in my death recaps (like 10k Swallow Souls).
Just to clarify though, I'm not 100% sure about the lack of the dmg mitigation, because I'm PS4 and without the on-screen numbers-add-on thingy it's hard to tell, since I have to rely on death recaps. Not to mention that not every player I get to fight is maxed out, so it gets really hard to say if he/she's exploiting damage to the limit or it's just someone hitting me for full damage.

What I'm 100% sure though is that I don't have the passive on my charcter sheet:



Some clarifications would be much appreciated, I'd really like to know if it's actually possible to pull those crazy values (I might as well post some death recaps if I can)...