When reloading the initial autosave of a game, I sometimes found myself in this situation, with no cultural borders at all :
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It seems linked to the fact that your capital city is not founded, but instead flipped from an existing one. As you have no borders, you also have nowhere to put your citizens, which means you'll lose 1 Population in the end turn, before the borders go back to normal. As shown, the game seems to think the borders have expanded, but... clearly not — there must be some check that works on the normal run of the pre-game, but not on loading the associated autosave.
Not particularly noteworthy or important, but still worth mentioning.