Today, I ran Explosive Conflict in 8-man Nightmare mode, since I was hoping to get the achievement now that it is fixed.
The patch notes read:
Players now granted the proper Achievement upon defeating the Firebrand and Stormcaller Tanks in all difficulty modes of Explosive Conflict.
After defeating the Tanks, I incorrectly received the achievement for 16-man Story mode, and I am still missing the one for 8-man Nightmare mode. So there still is a bug there.