Morellonomicon gameplay bug

When playing in a match, (I've only bought it on ARAM, Poro king and Dominion) if you purchase and by themselves, then upgrade to morellonomicon, For around 3-10 seconds you get free 10% bonus cooldown. So say I have the two, which gives 20% CDR, Then upgrade to Morello(Which shouldn't increase CDR anymore) you go up to 30% for a few seconds. This is just a minor bug but I tested and found that the CDR given for the few seconds DOES effect gameplay. Be it a karthus ult from spawn, soraka heal or a crossmap ultimate, they get the cooldown reduced if used while it jumps up the added 10%. Thanks for your time!