Hello -
In case you are still trying to solve this (one year later), here is something that you can look into :
The default Epic Male skeleton has some extra “twist bones” in various places - legs, shoulders, wrists. These can sometimes be used to counter-rotate areas that might deform too much on a model when in extreme poses. However, most third party animations (Mixamo, mocap) converted to the current skeleton will not drive these bones.
Your video seems to show that you somehow ended up with an animation asset that does drive these bones, but in an unintended way. So the first thing to do would be for you to look at the skeleton tree, locate the forearm/wrist twist bones and rotate them to see if they do indeed cause this floating wrist deformation. From there you could then look into finding a way to prevent that. These twist bones probably received some unwanted data from the retargeting process, so it is very likely possible to fix by retargeting the animation once again but this time making sure that there is no data retargeting going on towards the wrist twist bones at all in the retarget manager.
Good luck !