I’ll just chime in and say that the standard fbx works just fine. If you don’t feel like updating everything, UE4 provides pretty advanced retargeting tools.
I don’t think the root and twist bones from ue4 ever worked with the mixamo rigger did it? Those bones had to be added after the fact, Since you rig it and then you tell the site to download it in a certain format, like ue4 skeleton compatibility.