Loading earlier worlds, saving it and loading it again, some chunks are missing

When I load worlds from earlier version of minecraft (My earlier world is from version 0.14.1.), the world can be loaded successfully without any problems. But when I save it and open the world again, the problem happens: some chunks are missing forever! I have two earlier worlds and I all try. They all have the same problem. The problem I have shown in the screenshots (before saving and after saving).