Pockets ulti not causing assist to proc when dead?

[Osiris]

Question here: You dont get an assist if you ult as pocket and they kill the person a while later, BUT still taking damage from ulti? The ulti lasts for 18 seconds by default but I didnt get an assist on a teamfight, they killed them about 15 seconds later, so still taking damage by my ulti, but I was dead. No assist, is that planned?

 

[Osiris]

I believe it was MATCH ID 4309250. Although I didnt have a chance to look at the time