There is no "unwritten rule" just wishful thinking. You took the NPC fair and square. If they wish to contest that, they can put their army up against yours and see where the cards fall.
But, if they insist on something, then simply teleport the city just before the reset, take the valley after the reset, and gift him a level 1 NPC instead. It's the least you can do. ^.^
But....the eternal ray of sunshine REALLY wanted to see you, so I helped brighten your day by removing the city from around you! ^.^
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