Sion passive on-hit bug

When Sion died, the passive should still proc Tiamat AOE damage for auto attacks, but during this game, sometimes the Sion passive did not do AOE of Tiamat when autoing, say minions or champions.

In the video, it shows that once I died, the Sion passive autos do no hit surrounding minions, even if the active of the item works fine. The Tahm Kench also had a Tiamat but his auto has the AOE.