I want to clarify that i have multyclass character: Cipher(soul blade) + Rogue (Assassin)
I have found more strange thing: if I upgrade Soul Whip to Draining Whip there is no bonus (20 %) to my character's attack, but if I choose Biting Whip - miracle! attack bonus is displayed (30%).
I have checked this point: it's really strange!, as you can see on screenshot there is no bonus from "Soul Whip":
but it's just first attack in the beginning of battle. Second attack calculated with "Draining Whip" attack bonus and this one is displayed in character info now:
No, it's not my case.