Not sure if this is related to the ‘Passive’s firing on wave nodes’ bug that is already documented or not. I’ve only seen this during the Sinister Six event.
It does not happen every time, and I haven’t been able to determine what causes the error, because I can’t reliably reproduce it. Occasionally, the order of operations from one wave to the next gets jumbled.
For example: while fighting Goblin, I deal the final hit, and he immediately drops off screen and is replaced by Sandman. Then the damage calculation animation shows and the banner pops up saying that Sandman is downed. I also saw a banner showing the activation of Sandman’s passive that reduces damage by 50%. This is the only time I saw that banner pop up.