I think it's an animation problem...
Basically Slayer prevents energy drain while Bone insta drain ... Now both proc the effect as the match start but Slayer got a 0.5 sec animation, and this make her avoid the initial Bone drain dmg that got no delay ( prob the stand is immune till placed )
The same kind of interaction occur with Horse-Bone ... Horse should completely deny Bone, but he got cast time and so he can't prevent bone initial drain
Adding an animation to Bone's Fear skill will prob fix the Slayer issue but wil also make her completely unusefull with Horse around