Very happy that it’s useful! 
Nothing wrong, simply (and unfortunately) Mixamo animations don’t encode proper root motion information so they can’t be used with “UE4 root motion system”. By default the character motion is encoded in the Hips bone, unfortunately added to the local Hips motion, making it unusable. We also tested the “in place” Mixamo animations (you can export them from Mixamo flagging the “in place” option when available), but also this is not compatible with “UE4 root motion system” (it zeroes the motion - so any useful information is stripped out).
What you can do with our plugin, is to apply an existing UE4 root motion animation to a Mixamo character - this will work as expected thanks to the addition of the root bone.
workaround With another user @Konflict - thanks!) we tested a partial solution - but it must be tuned for each single animation and it could produces low-quality results (model could “slides” following the local hips/pelvis motion - this because it’s impossible to separate the local Hips motion from the character motion): use an animation blueprint that copies the pelvis/Hips translation to the root bone as needed, then “zeroes” the moved pelvis/hip transforms. In attachment you can see some screenshots of this method (credits to @Konflict).
