Just some news I have discovered.
As of today (16/12/20116), Mixamo has dropped support for Unreal Engine specific .FBX downloads.
This means if you have existing UE .FBX skeletal meshes, you can no longer download new animations and target them to that character.
You will need to use the standard “FBX” download and replace the older character entirely.
It will still work as expected, the key difference here being that root and twist bones are no longer available in Mixamo downloads.
I am not 100% sure on what these two features are/were for, but i’m sure someone can elaborate more on this.
More information on this change here.