Fairness does not guarantee everybody leaves equally happy.

A genuinely fair outcome distributes disappointment. Since people arrive wanting more than the situation can give, an arrangement that leaves everyone fully satisfied has usually taken from someone who was not in the room to object. Using happiness as your measure of fairness rewards whoever pushed hardest and punishes whoever was reasonable.

When you are judging a decision, ask whether the reasoning would survive being explained to each party in turn. If it would, the unhappiness is a cost of fairness rather than evidence against it. If you find yourself explaining it differently to different people, that is the actual warning sign.