When the Warring Clans assault is on, the Thundering Pandaren Spirit phases out when you get close enough to pick up the quest.
It is right on the edge of the Vale and Kun-Lai Summit. If I move into Kun-Lai he isn’t visible at all. If I move into the Vale he appears, but moving close enough to pick up the quest means he phases out again even though I haven’t shifted into Kun-Lai.
Edit: I spoke with Zidormi to change the Vale back to pre-BfA and it made no difference. The phasing issue still occurred.