Hi Mate!
Basically for a skeleton to be useable in Unreal Engine, the entire Skeleton needs to be parented under a single root bone structure, otherwise it won't work or not even be able to be imported into a project.
This one seems to be split in 2, with the arms and legs being two different skeletons and not merged into 1 (and have that single root bone between them). So the Skeleton Mesh (the model) uses only the legs, with the arms remaining stuck in T Pose. Somehow the Arms are stuck to the bag.
As is I wouldn't recommend this for Unreal Engine.
The Mixamo Skeleton is indeed compatible with Unreal Engine for future projects :)











