The bug here is obviosly not that Evelynn cant cast her ultimate while dead, but that her ultimate animation and soundeffects will actually play out while the ability deals no damage and doesnt go on cooldown.
This bug can easily be recreated by damaging Evelynn to low health and making her use her ultimate just right before she dies.
She will then proceed to cast the ability, showing its animation and creating the sound effect, while not dealing damage, not warping backwards and making the ability not go on cooldown.
It takes good timing, but when executed correctly, this will happen at least 7/10 times.
So far it seems that especially ranged basic attacks and damage-over-time effects (like ignite or Malzahars E) trigger this bug.
The easy solution would be to either allow Evelynns Ult animation AND damage to go off while she dies channeling the ability or to make the animation and sound effects not go off when same happens.
At the moment though, it just leads to very confusing sitiuations for both the Evelynn player and the enemy, since this Bug is not happening uniquely to the Evelynn player, but can also be seen by other teammates and the enemy team.