Since 9.2.5, the Undercity Flightpoint (as offered by Michael Garrett in Undercity for Horde characters) cannot be collected by ATT because there is "No flightpoints connected to this one", causing ATT to not be able to cache this.

I suggest this being temporarily marked as unobtainable until such a time it is fixed, which seems to be due to phasing.

There is some extended commentary on Wowhead that dives into this a bit deeper:
https://www.wowhead.com/npc=4551/michael-garrett#comments:id=5381402