I died while fighting Sheliak's Yoke and went back to my respite realm to repair my gear. When I returned to the heart of the realm, the giant bug was gone. I tried logging out and back in, but the boss is still gone. Not sure what my next move is. (It's a great game though and you guys are working really hard!)
I didn't try the pillar trick, just reset the realm and fast-tracked the trip back down the well.
Destroying the pillars with the pick made it respawn! Not sure if this is a feature!