Is there a bug with power strikes? or a ninja buff? because it's randomly coming off CD WAY earlier than 15s.
It has a running clock that checks every 15s and puts it up if it isn't. So, if you used Jab 10 seconds into the respawn time, it comes up again in 5 seconds.
What you may be noticing is the fact that the timer keeps ticking in the background even when the buff is on you. It's a little safety measure to help make sure you don't get totally screwed over if you have to delay an attack for whatever reason. So you may have the buff, be sitting on it for say 5 seconds, jab, then 10 seconds later it will come off cooldown. If you then jab immediately to clear the buff, it will reappear 15 seconds from that point.