I know basically feck-all about this but I happened to watch some of this fella’s amazing tutorials recently and he gets this problem when using Camtasia:UE4 Tutorial: Basic Movement with Root Motion - YouTube
You can look there and see what other clues you may find but as I understand it, exporting the animation with Camtasia running will cause this to become permanently part of the exported animation, and I think having Camtasia open whilst attempting to play a perfectly exported animation in UE4 will cause the animation to play back incorrectly. If you don’t use Camtasia, that doesn’t mean some other software isn’t affecting you. Try closing everything but your modelling software and UE4. If that doesn’t work, try restarting your PC and then opening only your modelling software and UE4. If that’s not it, I’m unfortunately not going to be very helpful. Best of luck.