LOTS of us had issues this week:
It seems to only affect characters who did this quest LAST week. If you do it on an alt that did NOT complete this TW quest last week, you will probably be able to run the dungeon and get credit as normal.
Some have been able to queue through the Adventure Guide and keep the quest (I was able to do this on 2 of my 6 characters). Portals/getting loading screens/etc. seem to increase the chance of losing the quest.
Blizzard is aware of the issue: https://us.battle.net/support/en/help/article/370716
but they obviously have not fixed it so…