There are a few other threads on this. All very old, as this has been an unfixed problem for three years now. The most confusing thing I’ve found is video footage of all four flight variants from the BfA PTR. Meaning, the art was done, coded, and functioned adequately on a playable server. Then all but one variant (the brown one) was left out of the live release.
To any Blizzard rep that may have stumbled upon this, if this is a bug can you pass this info along… and if it is not a bug, can you still try and do something to get these missing forms back in the game?
Below is a link to another post I made that contains links to pictures and video of the four variants in question. They look amazing. Don’t let them go to waste!
Kul Tiran Flight Form - Support / Bug Report - World of Warcraft Forums (blizzard.com)