So... uhhh... the game broke. Rather amusingly, because it seems like such an obvious oversight it should never have been missed, but still annoying. And when the reason something is funny is that it seems impossible NOT to have realised it was a potentially game-breaking problem, that's... well, a potentially game-breaking problem. Here's some steps for reproduction of the problem:

1. Send an exploration ship to claim an unclaimed island.
2. Get unlucky with an early viking raid slaughtering everyone, or killing enough people that it can't recover before the next raid.
3. There's no way to send more people to the island, and even though everyone is dead, it still counts as "settled" (even if you make a point of destroying every structure that was there). Once it dies, it's dead forever, no way to recover it.