Sinkholes act as wormholes to a new world, so can't access caves

It looks like you've run across a very rare bug related to custom world presets. It's on our list of things to fix. In the meantime you can "fix" the problem locally by deleting all your worldgen presets and recreating those that you still use.

Unfortunately there is no way to fix that particular caveless world. It was created with a second "regular" world instead of caves, and that can't be changed at this point. 

That sounds correct. Thank you that helps.