Guild Wars 2

I went into the Mad King’s Labyrinth on my level 80 elementalist, on whom I already had 10% progress towards my Revered Antiquitarian Mastery, intending to level up said mastery. When I got into the Labyrinth, however, my mastery bar read 0/0. Upon killing several mobs, I noticed it hadn’t increased, and yet, my combat log says I had gained xp.
When I go in on my Rev, she gains XP as normal toward her next level, in fact I earned several levels this way.

The Hallowe’en instances don’t give Mastery XP. This is a bug, but it’s been announced that they won’t fix it this year.