Update 2: the fix is now live on all realms, so XP gains should be back to normal!
Update: Blizzard have shared that the XP reduction was not intended on their end and that they are looking into the issue:
It seems Blizzard have turned the XP nob down a little ahead of schedule! Players are reporting that the experience gains have been significantly reduced in the pre-patch event. This started as of around an hour before Early Access launch.
Many reddit threads are talking about the reduced XP rewards, as it seems the events are no longer giving experience upon completion. The mobs in the events are still awarding XP, but you’ll have to get a tag on them to get it.
The consensus seems to be this was done to prevent early leveling shenanigans once Early Access goes live.