Using a Sleep move always fails, even if the target is airborne. However, sleep moves work if the user is airborne, which is not how it's supposed to work.

Used a Tapu Koko with Spore to test this. In Electric Terrain, Spore fails against the Pidgeot, but as soon as the Tapu Koko has an Air Balloon, it works. It's not supposed to be like this tho. Sleep moves should work against Flying targets no matter if the user is grounded or not.

Thank you for reporting. Some issues with terrains and non-grounded pokemon are known, but this specific issue hasn't been brought up before so I'll pass it along.