There are two reports (me and someone else) of using Plague of Arrows on a character/minion and killing them (or not). Both reported a character with 0 health still being active.
In my case, I used Plague to kill a toon and the next (final enemy) toon had health. I used a match 3 and they lost all their health (down to 0). I could not down them with match 3s and had to use an ability to down them and win the match.
Yes, I just had TWO opponents in PvE operating with 0 health and continuing to cause both match damage and ability damage to my team! I was only able to down them by firing another power that caused damage. I was playing Pestilence, Famine, and Galactus vs Capt Peggy, 4* Deadpool, and Venom in the current Welcome to Shield. I downed Peggy but the other two kept going even with 0 health.