IKRetarget Strange behavior

Hi,
I’m trying to retarget an animation from UE4 mannequin, to a similar skeleton with different proportions.
As you can see on the video, fingers are longer.
Both IKRigs are set with fullbodyIK, and have standard foots and hands goal, nothing fancy.
Yet hand’s goals are making phalanx offsets for no reason … (beginning of video)
Then when i disable IK solver, it seems to be good again.
However, when i export the new animation, both hands get stuck in the shoulders …
Is this known? what the solution? did i miss something?

regards