Didn’t see any mentions of this, hope it’s actually a bug.
Since the 4.6, it seems like Sienna takes the 100% health damage from reaching max overcharge after / at the same time as the damage when being downed, instead of being the cause of death, or being negated by getting downed before the animation ends.
This is quite easy to reproduce in-game by picking Unchained with the Abandon talent. Everytime i have been downed as Unchained recently while being in the max overcharge animation leaves me on the ground with ~30 hp in bleed out with a visible drop on the health bar.
Didn’t test this with Pyromancer and BW, since it’s harder to set up, but if it’s overcharge related, might still trigger.
I think it’s safe to say that it’s the overcharge mechanic in general that seems to apply its full damage if you’re being downed as the overcharge animation is in progress. At least it definitely looks like it.
Just a guess, but if it affects the overcharge mechanic in general, it might also affect kerillian’s staff. Just writing this down so the devs also test this case.