This is more of a design oversight than a bug, but it is quite a nuissance. Worldgeneration sometimes places the biome with the Aporkalypse calendar ruins behind a pond biome, meaning you need a boat to get there. If you are not wearing the necklace as Wilba when Aporkalypse starts, you're stuck as a werepig till the end of it, so you can't drive boats and therefore have no way of ever getting to the calendar, which softlocks you in an eternal Aporkalypse. If I'm just overlooking a feature that can get Wilba over the pond, please tell me, but right now it doesn't seem like it.
Play the Steam Version with all DLCs enabled, create a Hamlet world with calendar ruins ('pig_ruins_entrance5') generated on the other side of a pond, pick Wilba, turn into a werepig during the first Aporkalypse and then you cannot cross the water to get to the calendar.