Afaik the movement speed reduction was larger (if I remember correctly it was either -35 or -45 movement speed at level 15), but it was “adjusted” (i.e.nerfed) with a more recent patch to give only 1 movement speed reduction per skilllevel.
Thta’s because it used to stop monsters and players if you were able to reduce their total movement speed to 0(not worth to nerf the amount,though, as this wasn’t that gamebreaking in most instances, only in TBL and ET you were able to have a decent advantage).
Anyway, it was not a bug but another “silent” ninja patch we recieved at iTOS some weeks ago(since we usually don’t have well-made patch notes unless there’s a larger & compiled update incoming).