Most chunks generate void below y=0 on old worlds

After seeing Minecraft Live, I wanted to see what my old Xbox One Edition world looked like below y=0. So I transferred my Xbox One world to my PC and then to my Google Pixel 4 via FTP, started up the world, went to my mine, dug below the Deepslate, and noticed the void below. Then I looked further, and saw that some chunks actually did generate below y=0 (as seen in the screenshot)