Guys. It's probably not a bug. Not saying certainly not a bug, just thinking about it aloud, but hear me out:
The wording for the achievement is: "Complete the Kul Tiras and Zandalari achievements listed below". This can perhaps apply to just the Explorer, but I think if I were them I'd say "or", or use a different wording.
The number of achievements is just five, removing two leaves three, this seems very small. They'd likely pad with some other achievements if they were to remove two existing ones.
The logic for requiring achievements from both the Horde max-level and the Alliance max-level is this: "Since you are getting increased mount speed and then flight in BOTH Kul Tiras and Zandalar, you have to experience them both, and if that requires two separate characters, well, so be it."
And they keep silent about the achievement while being asked continually. Perhaps because they didn't realize that this new requirement is not going to be received well.
So, yeah. I'd like the official statement, but I am getting the feeling that two chars are the new requirement. (And yes, this is terrible. What's next - the requirement to complete all mythic instances on 15+? Have a max-level of each allied race?)