Yes the extra-bones should be ignored by UE4. Now I’m quite sure that the problem is the joint rotation. I modify my software in order to show what format unreal use as default.
It exports animation as Maya/3D Studio Axis System (Z-Up, -Odd, RightHand)
UE4 do not care if the imported animation is in Z-Up or Y-Up axis system but for sure it do something with the joint rotations and bones.
In order to test it I exported the default Mannequin Idle animation to an FBX and imported it into my 3D software, then I saved 2 new copies with Z-Up and Y-Up axis system.
As you can see from the screenshot UE4 can reimport the animation and everything works great.