Players with XP disabled are joining the War Within pre-patch event, reducing XP for others. Luckily, a WeakAura helps identify them!
Introducing XPEliminatedFinder, a WeakAura that alerts you when someone in your group has Experience turned off, impacting your XP gains.
But why would anyone disable XP during the event? The reason is straightforward: certain transmog pieces from the pre-patch event become unavailable once you reach Level 70 and to get them, players freeze their XP.
Hopefully, Blizzard will look into this before the event is over.