Yeah, this isn’t currently supported, but I would definitely not call that a bug.
Perhaps the markdown interface is too permissive and that’s misleading and you might well argue that’s a minor bug. But it shouldn’t be a surprise to people who know instant run-off that this isn’t part of the algorithm.
It’s not really a standard feature of Ranked Choice, Instant Run-off Voting, where the voter has the discretion to select the number of candidates they wish to rank, and can abstain on the others.
Imagine restricting someone on a ballot paper. I guess someone could call your paper spoiled if you rank 4 out of 5 candidates and somewhere on the instructions you were told you could have only 3, but suspect that’s not usual as it would cause confusion and a lot of spoiled ballots which might upset the electorate.
But of course Discourse doesn’t run on paper 
And sure you could definitely add that feature! - it could make sense to limit things to 2 or 3 choices out of 5.
See: