To clarify the issue:
I was able to turn XP off and back on with a random character I had.
It seems as though the buff you receive when you turn off your experience is currently missing if you had your experience turned off before the patch.
My guess is the game doesn't recognize you as someone who can turn off your XP because it is already off. Unfortunately, the NPC doesn't seem to recognize you as someone who can turn on your experience either because the lack of debuff.
That said, as a curious turn of events, it doesn't seem to be giving party members the reduced experience gained debuff, either. I suppose this could be seen as an advantage in some regards.
EDIT: I was able to fix this by partying with a character on my second account and disabling/enabling XP with the broken character present. For some reason, my low level character getting the reduced XP debuff was enough to have the game check for whether or not her experience was enabled.
What a weird world.