The samplerate of a track was compute with the following formula
Equation 1: number of Key / animation Time
This formula work only if we have animation keys that are time equidistant.
example of non time equidistant key:
239 keys in the first second and just 1 key 9 seconds later. This will give 24Hz but it is not
To fix this issue we have calculate the least common denominator of the Keys to find the best samplerate up to 30Hz that will go through all imported keys. But this algorithm is not working well with time equidistant keys. and we use it even with time equidistant keys animation.
The fix is to use again the "formula 1" if the animation keys are time equidistant.
The issue was found by one of our client in the 4.17 preview forum see the link
import the attach fbx with the animation with "anim time" and uncheck "default sample rate"
EXPECTED
48 frames in 2 seconds
RESULT
60 frames in 2 seconds
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-47342 in the post.