Zyra: Ult cool down not reflected in UI

I played a game where I was just starting my ult, but got dove on by a talion and immediately died.

My ult seemed to work ok, but the button on the interface was still active as if it was available. When using ALT-click on the R the cool down was displayed (40+ seconds) but the button itself continued to look available.

So the ultimate had activated, but the button never greyed out.