I had the same issue and this post finally helped me fix it: https://www.reddit.com/r/wow/comments/1f25myy/experience_never_turned_on/
- Join follower dungeon
- Wait ~30sec till you get a XP debuff (I’ve never noticed this before so I suspect this is part of the bug)
- Once you have the debuff you can leave the dungeon and you should now see your XP bar and start gaining XP
I’m really hoping Blizzard realizes this is a bug and recovers the XP for all of us who hit this bug. I missed out on all XP for the first few hours of TWW.