I was playing Karthus in an ARAM. I had 118 ability haste putting my ultimate on a cooldown of around 64 seconds. Near the end of the game, I cast my ult, but when I took a look at ability cooldowns, I saw my ultimate was ticking down from around 99 seconds instead of the 64 seconds I had seen every previous time I had used my ultimate at full build. I even moused over the ability to see the cooldown as displayed by the ability tooltip, and it stated the expected cooldown of ~64s.
Why did my ultimate gain an additional ~40 seconds of cooldown?
My build consisted of Liandry's Anguish, Rylai's Crystal Scepter, Ionian Boots of Lucidity, Cosmic Drive Void Staff, and Rabadon's Deathcap. I had an Elixir of Sorcery active, and my runes consisted of Dark Harvest, Cheap Shot, Eyeball Collection, Ultimate Hunter, Transcendence, and Gathering Storm. The enemy team had a Xayah, Malphite, Zilean, Viegar, and Brand. My team had a Kindred, Lucian, Teemo, and Jhin.
I did not sell any of my items. Once I had reached full build, I would purchase an Elixir of Sorcery every time I was able to purchase from the store. The amount of time between my deaths was not long enough to have the elixir fall off either.