The game generated a new volcano, deleting the prexisting one (same with ROG's caves).
Am not sure of how it happened, but perhaps its using the teleportato on any of the worlds what causes the subworlds to reset? (I used hamlet's teleportato before the caves and volcano reseted) I will try to confirm later.
Mods: Only Skin mods and Animal variety, none of them affect worldgen.
EDIT: CONFIRMED. Using hamlet's teleportato reseted the caves when it should only reset the hamlet world,probably resets volcano as well. Some research indicated that this bug has been around since SW, which means that using any teleportato will delete the other world's subareas.
Steps to Reproduce
1)Explore caves/volcano 2)Go to hamlet and use the stone thing 3) come back to caves/volcano and the game will generate new ones
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.