I was playing Oscorp: Heroic, battling Bullseye, Moonstone, and Ares. Ares used Sunder, which in addition to damaging my character, inflicts damage on himself. In this case, the self-inflicted damaged downed him. However, the yellow countdown tile that the power generates stayed on the board, instead of disappearing when he went down. All other characters’ countdown tiles vanish when they go down, so I think this behavior is in error.
Same thing just happened to me too. I killed him after his sunder, but his tile stayed.
Sometimes it stays and detonates. Other times it disappears. I haven’t figured out why yet. . . 
The sunder bug is there from first intro of the character and was reported a zillion times, still not fixed.
Guess all resources are tied up to invent new ways to punish those still around.
I also experienced this. Same mission, too. I reported it.
When this happens, an AOE attack will clear it. It is almost like the AOE re-checks and says “yup, Ares is dead, clear the tile”.
Good thing i looked first i was just about to add this, Lol
I don’t frequent the forums enough to recognize “AOE.” What does it mean?
AOE stands for “Area of Effect” i.e. attacks that hit more than one target or hit the whole team. So attacks like Thor’s Call the Storm, Storm’s Raging Tempest and Wind Storm, Black Panther’s Rage of the Panther, etc. all count as an AOE attack type.
Also see here for more Ares Sunder issues: