Question about Retargeting on Custom Rig

I’m trying to retarget thisanimset to my custom rig (which is similar to Epic one). This pack involves animations with a two handed sword which you need to attach to ik_hand_r.

When I try to retarget the animations on my character (which doesn’t have ik bones) everything looks good except for the weapon movement (watch the video).
I found out that the problem is the hand movement which is deformed compared to the original animation which causes the behaviour you see in the video but I don’t really know why this is happening.
It’s also quite obvious that something is not right with the right hand retarget because the wrist is twisted. I thought that the problem was that I didn’t have ik bones but on Epic skeleton the movement of the weapon is perfect even If I attach it to the hand sockets ignoring ik bones.