Karthus Ult timer bug.

Today I was playing League of Legends (legends of the Poro king and I was playing Karthus. During the game I was playing every time I used my ult the timer would be incorrect therefore I couldn't use my ult. Regardless I still won the game, although I would have to wait double, almost triple the time to use my ultimate. I'm not too sure if this was intentional, but it makes it really hard to play to Karthus being that his ultimate is a big part of his kit. Hopefully this bug isn't just me and this problem can be resolved soon because Karthus in Legend of the Poro king is very fun and I enjoy playing him, thank you!