[4.7.6] blend poses cause mesh morph

I noticed in the anim graph as I use more and more layered blend per bone, aim offset…etc, to blend my animation,

my character starts to morph. The morphing increases as the blending number.

What is causing this issue? And how can I solve it?

Further notice:

I’ve used blend per bone in UDK, but it only effect the bone translation/rotation, but in UE4, it seems like it is trying to “add” all the animation together, which is not what I expected. Is there a way to over come this issue? The same also happen to aim offset, but I can solve it by /translate/rotate bones individually, however, currently there seems no way to get the blend per bone correct without suffering from a degree of morphing.