When stretching a FFD/motion clip in time(making it's duration longer), the underlying keyframes are not interpolated correctly. The result is a stuttering motion instead of a clean interpolation.

For example when I make the FFD motion three times as long as the original, it does not interpolate and instead just uses one frame three times in a row.

Looks like this: (4 Frames streteched by factor 3)
1 2 3 4 >>>> 1 1 1 2 2 2 3 3 3 4 4 4

The result is obviously a stuttering motion.

Video and project file in private comment.