Hi, i had some issues with UE animations and the new UE animation retarget system.
To have animations retarget correctly make sure that the new target looks exactly as the old one.
Example
While this seems obvious, it might not be, when you deal with 2 UE skeletons, and changed the default pose. The current pose will affect the retargeting, even when otherwise deemed compatible. Thus, you need above retarget pose currently set, otherwise animations will be off. Also for some reason you need to retarget animations, matched to the default UE character, ie. all marketplace animations need to be retargeted to your current inuse UE skeleton.
Maybe this clears up some of the confusion about retargeting.
Also if you retarget animations from characters not based on the UE skeleton, you have to select Humanoid Rig inside the retarget manager, and a default reference pose, which matches above UE base reference pose.