Pretty simple deal, though I suspect there's more to it. Void Assassin's Wash Away can remove poison even when the target is under the effects of Menov's Fang. Noticed it in a spar, but I'd assume if that works, things such as Llunel's Decay and whatnot would also not work against Wash Away. If anyone wants to get a buddy and try to see if they can reproduce it, please do so. I don't have friends, so I cannot--
Wash Away, like Negotiate, is a dispell effect on yourself that removes negative buffs at random. It's not a specified poison cure like in Martial Artist and Black Knight's cases.
Those things are just the gimmicks of Rogues, so I don't believe it should be changed.
As a note, and not simply a suggestion or opinion, the following methods may remove non-curable status effects such as hexes:
-Cleanse Potion
-Negotiate
-Wash Away
-Spirit Jar
-Plant Rejuvenate
-Golden Blood
This makes the issue presented not exactly uncommon to face.
Negotiate has been corrected. The other effects are duration reduction, which isn't curing the effect.