A player can perfectly to have as a strategy to buy planes before he needs them, & limit the lease time. So that, when he needs them, they're back. Especially if he plans a "big bang" fleet replacement to avoid the 4th fleet penalty. Did you lease all your planes to the same player? Not a bug.
In Beginner's world, though, used planes should be easy to find, and you'll probably find enough to fill the gap. And it shall be a lesson for when you promote yourself to "real" games : beware time limits when leasing planes to other players.