[GAMEPLAY] Karthus ult cooldown not reducing right

In my previous game, I as Karthus was forced to endure longer cooldowns for my ult. At the end of the game, I had to wait ~160 seconds for my ult to cooldown despite the description saying 88 seconds.

Runes: Arcane Comet, Manaflow Band, Transcendence, Gathering Storm, Future's Market, Cosmic Insight.

Endgame Items: Boots of Lucidity, Luden's Echo, Rylai's Crystal Scepter, Liandry's Torment, Void Staff, Rabadon's Deathcap

Video: https://www.dropbox.com/s/8mkjcn8vgzz213d/NA1-2821893920.rofl?dl=0