AimOffset breaks a skeleton

Hi,

I am trying to create an aimoffset based off Kubold Cover Animset. Specifically based on 3 poses: CoverHi_AimL_L, CoverHi_AimL_CC and CoverHi_AimL_R.

Kubold made ik bones (ik_hand_l and ik_hand_r) of the UE4 mannequin, sort of, attach to hand_l and hand_r bones respectively. The thing, for instance, not implemented in default animation sets from Epics.

Now the problem is that in the aimoffset those bones are detached during the transition between poses.

Here are the screenshorts:

Here bones are attached

This is the transition - bones are detached

Attached again

Now here is the question? Is that a UE4 constraint? How can I workaround it?
If no, then what did I do wrong here?

Thanks

p.s. I made a gif to illustrate the problem