I have recently re-discovered a survival world (vanilla, no mods) from 2012 that was at the latest played on 1.3.1. I am not sure what exact version I created it on. It is later than Beta (there are jungle biomes), but the large oaks still have "upright" branches so somewhere between Beta and 1.3.2. I have attempted to update it to 1.16, however when I do so, some chunks here and there will reset to 1.16 terrain. I have experimented a bit and if I load the world in 1.12, the original chunks I visit don't corrupt. However anywhere I haven't physically loaded in 1.12 is at risk from chunks resetting to 1.16 chunks. I do have backups. My main world is my first world from Beta 1.6 and I currently play that world on 1.16 and I have never had this issue on that world so I am a bit confused. I wondered if anyone knows what is causing this? I can just try and visit every chunk in 1.12 but it is quite a big map and I wondered if there is a better solution.