Lissandra Bug/Frozen Heart Bug (Not really sure which one)

So i spotted a bug in game just now (at least i think its a bug). I was playing Lisandra and when i died i bought a Frozen Heart. Before i buy it i have 25% cdr, then when i buy it it jumps me up to 45% cdr but after a moment my total cdr drops back down to 35%. I didnt exactly have a recording software but i signed up for replay.gg and if you look at the replay at 21:30 my cd was 97.5 Then i bought the frozen heart at 21:50and my cd was at 71.5. Then like i said it drops back down to 35% and my ults cd goes back up to 84.5 at 22:00. You can tell my cd by hovering over the little circle next to my name on the side bar btw. I was wondering if you guys could help me out by finding if this was a Frozen heart bug or a Lissandra bug.