When copying a keyframe from one cell frame to another, Velocity remains at 100% in the Function Editor.
This happens on the third keyframe and then all subsequent keyframes added.
Make sure the Velocity keyframes are readjusted afterwards so that they are always ascending to 100% from the first keyframe to the last keyframe.
A static velocity between 2 points will give awkward timing results since in reality it would mean the element needs to be in 2 positions at the same time.
Mac OS X (all)
Toon Boom Studio 5
Toon Boom Studio 6