Members of my guild were in Maw of Lorkhaj, in the room before the final boss, when we were all booted from the game back to the character selection screen. When we logged back in, we were all outside of the instance, and it had reset. I submitted a support ticket, and was told that I should post about this on the forums as well. Here is a video that shows what happened:
https://youtu.be/gTmy1q2gnzE
The platform is PS4. This happened on 03/26/2016 at approximately 11:00 PM Eastern (UTC-4).