I had the same issue, but I’ve solved it here like that:
In the Retarget Manager window, check View Pose. When I clicked mine and set “View Pose”, the hands were totally deformed. So I clicked “Hide Pose”, then Modify Pose> Use CurrentPose, and clicked “View Pose” again. The hands were fixed. When I retargeted animations to the body, the hands/fingers poses were fixed.
I’ve tried other solutions and nothing hasn’t worked for me. This was the only method that worked for my MetaHuman.