The attached fbx was exported with dense data, i.e. there are 10 subframe keys set per frame vs the usual single key per frame. We do this to eliminate subframe "jitter" that occurs sometimes with weapon bones or in this case a long joint chain for the Hero's whip.
When this fbx was originally imported into the editor, it was possible to scrub the timeline in persona and see that that whip animation played smoothly and the NumFrames showed 491 indicating dense data on a 50 frame animation exported with a sample rate of .10. Currently, if you attempt to import this same fbx into the editor (which I tested by importing it into a new folder so as not to overwrite the anim) you will find that the dense date is NOT imported. The NumFrames column shows 50 and if you scrub the anim in persona the whip joint chain pops and jitters around excessively.
Any animator working on weapon animations that involve 360 rotations or otherwise complex action paths needs the subframe feature to work correctly.
see description
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44685 in the post.