Amumu Ult HUD Bug

As you can see from the imgur image i have linked, my ult was bugged. Basically what had happened was that I cast my ult a split second before I died. The animation for my ult started but didn't finish. Which resulted with my ult saying it was on cooldown. However, when I held alt and clicked my ult on the HUD, it would say in the chat that my ultimate was ready. Later in the game I just assumed it was a HUD bug with my ult being on cooldown so I jumped into a fight, assuming my ult was ready since the game said it was ready. It caused a lot of confusion in my team because they could also see the chat saying my ultimate was ready. This has happened to me on other occasions as well, but it was the complete opposite. My ult said it was on cool down, but I was able to cast it. This second occurrence I'm mentioning happened months ago so I can't get a screenshot or replay of that match.