it's part of update 0.7.3 that cap the nights at 12. Reasoning for that is that the devs wanted it to end at night 21 become of the moon cycle. If you are on the most current update and gotten past the cap, it's because the file from the previous update wasn't manually deleted and carried though. It's fine until you get too far, make the game a bit buggy. If you have any questions you could ask the devs on there discord.