I've noticed some odd behavior for the leaderboard of a set.
Usually, for a set of problems, you solve the problems "inside" the set - you click on the set, then you click on the problem you want to solve.
But you can also solve problems "outside" the set, i.e. you can get to a problem inside the set without clicking on the set. (Either the problem was already posted, or a problem inside the set gets reshared.)
What I've noticed is that if even you solve all the problems in a set, if you solve your last problem "outside" the set, then you don't get on the leaderboard. I feel like how you solve the problems of a set should not impact whether you make the leaderboard or not.