Lux R CD not showing when dying before animation finishes

To replicate the bug, kill Lux just as she is screaming and casting her R and no damage is dealt to opponent. Her ult will not show CD and cannot cast.

This happened to me twice. First time, returning to lane and was going to win a 1v1 with an ult kill shot. Ult didn't cast and so as I was running away, I was spamming R. It casted later when I was in the river.

Second time was just now and I question myself this time if my ult is on CD. Tested after I revived and it wouldn't cast.