Firstly - thanks for taking the time to help!
Yes, I initially loaded the vroid fbx model directly into UE4 (this is the pre-existing rig I referred to). I’ve applied animations created with Mixamo directly to it with no problems.
I did experiment with disabling leaf bones in Blender without success. However, Blender’s leaf bones do seem like the most likely thing to be causing the problem since the deformations happen in places where leaf bones would exist (fingers, toes and bust). So I’ll go back and try again.