Upon regeneration of my oldest world, me and a friend decided to try and build up. Upon hitting day 16 or so,I ventured into the cave to gather some extra resources. Upon my arrival,I immediately knew something wasn't right. The nightmare/cave cycle was nonexistent, using the above ground day/night cycle instead. Along with that many set pieces above ground spawned below aswell. Now I was curious what extent had changed, so I stuck around and explored to further notice the caves usual abyss was now comprised of water, birds and sea fish were spawning, and anything affected by the nightmare cycle simply did not work. Upon reaching day 21 I even noticed season's did not impact the caves, never once getting cold down there. Everything else seemed to work fine, underground set peices did spawn and the ruins seemed to be there.
Unknown but I have a theory. This world was regenerate from one of my first ever personal DST worlds five or six years ago. Now it was originally 500+, but my console had given out on my, prompting me to reinstall saved data from PS+ which rolled it back in the 90s. Once I hit something in the hundred, I gave up on the world and regenerate it. Until day 16, I didn't enter the caves so I think that may have caused the glitch.