I made a world in which i decided to get every advancement, so after killing the ender dragon for the first time I revived it with 4 end crystals. After killing it again, i remembered about the egg, and it wasn't on top of the portal, or neither near it (I probably hit it and it went through the portal). So I went to my house in the overworld (in which i had my respawn point), and it wasn't there neither. So I went to the spawn of the world, and again, couldn't find it. To see if it teleported to a cave or something like that, i checked again those 3 locations with spectator mode and x-ray, but it actually just disappeared, so, with no cheats, this bug makes the "The Next Generation" advancement literally impossible.