Stalker of the Endless Dunes stuck in invis / invuln phase.

http://xboxclips.com/Corrison/adfd8339-4ab7-438f-99fd-7942430e04ec

You can see the boss at the top of the center tornado. Waited around for a bit for him to come and attack us, they did not. One of us left back to hideout, he stayed the same. Both left and stayed out for 30 sec, upon re-entering boss was back to normal.

Not sure how or why this happened, but here is a video in case it helps..