[1.2.5 + Ashes 1.0.3] Bringer of Doom not giving achievement

[Velorien]

I summoned a Bringer of Doom on level 1 of the alt Sandworm Lair, then descended and completed the dungeon without killing it. I used the Rod of Recall to warp out, then went back in and killed the Bringer, but got no achievement.

I subsequently encountered another set of pedestals, and summoned another Bringer of Doom, in the Halfling Ruins. Killing that one did give me the achievement.