I'm not sure if this is only me or not, but I was on my Warlock, and my companions would either A) go underground
reset back to the start of the instance C) get stuck in combat or D) Disappear all together. I tried all 3 companions, I relogged, I restarted game, I rebooted PC, I rolled back to previous Video Card drivers as I had just updated to the newest AMD Crimson Drivers...nothing helped the issue. Funny thing is, I played the night before on my Hunter Ranger and it worked flawlessly, so I am unsure as to what the issue may be.