The Nether

So, It has been going for a while. In the nether if you tower up for a bit you will somehow insta die. It just happened to me again and lost all my progress. Can you PLEASE fix that. And It happened on Java. I was on a hardcore world that has been on for almost 2 months then I just went in the nether for some glowstone for a fishtank for my Blue Axolotl's then I randomely died because of that bug and lost my hardcore world. Please fix that