Easiest to see the problem with short animation sequences. So import an animation with time range 0 to 3 ( 0.1 seconds long if animating at 30fps).
UE4 will correctly display the animation length of 0.1 seconds.
UE4 will correctly say the animation has 4 frames. This is slightly counter-intuitive but basically you count frame 0 where no time has passed.
However if you open the anim in persona and look at the current frame a frame is added to the timeline which shouldn’t be there. For example in maya you may have a time range 0 to 3, if you import this you will see the time range now goes from 0 to 4. If you use the play back buttons to frame advance you will see that frame 1 is 25% of the way through the animation with a current time of 0.025. Frame 1 should match whichever package you animate in and should be 33% of the way through the animation at a time of 0.333. At 30fps every frame should be 0.333 apart.
I assume this is a simple mistake due to the slightly counter-intuitive nature of number of frames. It only seems to effect the frame displayed in persona all other timings seem to work. However it is slightly awkward when it comes to adding FX/sounds/notifies etc on specific frames. The error is pretty small, and gets smaller and smaller the more frames are exported, but for people like me who prize precision it is annoying.