Soloing Sir Blackwater, first hit (Glowbug) failed to kill storm minion. Storm minion had 9 health remaining.
In my next hand, I had Sirens. I hit with Shrike + Galvanic Field + 10% pierce blade. It's what I had, and I needed that minion dead. It obviously should have killed the storm minion. However, the storm minion lived and somehow had over 1000 health the next round. There were no heals that occurred.
I suspect this error is related to a larger game issue. This must not happen again.