MRV Picks The Wrong Variable

// MRV: choose the unassigned variable with the FEWEST legal values left
function selectMRV(unassigned, domains) {
  let best = null;
  for (const v of unassigned) {
    if (best === null || domains[v].length > domains[best].length) {
      best = v;
    }
  }
  return best;
}