RPG ammo takes a while to arm itself after launch (or at least western ones tend to be like that) to prevent accidents in case shooter hits a branch or something else in the heat of combat. But I agree, that looks like he used it correctly (and his arm positions support that, it would be difficult to push trigger wrong way around) and just missed. Ammo is just way too fast for drone camera, specially with that resolution+framerate.

Some variants might not even explode even if he hit the thing, but that’s irrelevant since a direct hit with pretty much anything bigger than a coin will bring a drone down.