I was coming up to the mid point in a Nightmare dungeon, and my alert went off for the World Boss. So I TP’d out of Nightmare, killed world boss, went back to Nightmare and it had reset.
I’ve had this happen on Bosses as well as Legions. I think it’s because you changed shards when you entered the boss arena, and that reset the dungeon. In my case, it actually changed dungeons completely.