This bug as been around for a while. Here are some other related threads:
https://forums.d3go.com/discussion/75516/ares-sunder-investigatinghttps://forums.d3go.com/discussion/67659/ares-sunder-countdown-tile-staying-on-board-after-death-devs-aware
I've noticed similar bugs from one of the Iso mutant goons: one of their powers does self-damage and creates countdowns. If the self-damage kills the goon, the countdowns sometimes stick around and don't tick down.