When you you world hop in a merged save, both the caves/ruins and volcano will be reset. Allowing thulecite to be renewable for example.
It is very unclear if this is a feature or a bug.
Steps to Reproduce
1) On a world with merged worlds, world hop in shipwrecked. 2) The caves/ruins rog are now reset alongside the shipwrecked world.
This issue has not been confirmed by a developer yet.