Doubt its a bug.
More likely due to the way Animations work. Its very likely that the Ultimate attack is triggered the instant the adds die. But it has an insanely long animation, and damage is applied at the END of animations, the problem is more likely that he is applying shields after the game actually applies the damage to you.
Same thing happens all the time with Tank Busters and Adloq.
If the cast bar finishes before Adloq lands, the animation will play out and the damage is applied … seemingly ignoring the Galvanise, but that's because the Tank already got hit by the Tank Buster before Adloq took effect, it just didn't 'apply' the damage until the animation said it should.
This is something we learned, as a community, a very long time ago ...