Anniversary Event Bug: Steelstar not registering if you kill him too fast

[Xgya]

Killing +Steelstar sometimes doesn't grant its star or clear the objective.

I found out by going at it slower that he's supposed to teleport away and then drink coffee before the fight ends.
If that doesn't happen and Steel just happens to die from sudden meteorological changes (sunny with high chances of Cometfalls), the fight ends, but you don't get credited for vanquishing him.

Edit: Steelstar confirmed speed has nothing to do with it, and he just sometimes spawns in a bugged way that makes the objective not complete.