World generates in flat worlds

It's often occured that when I make a flat world, two three days later when I restart my minecraft, the name of the flat world is erased automatically and when I go into the flat world, there has been an entire world generated around it, with a cut-off area of where I previously built my stuctures in the flat world. Look at the photo below. And please fix, I had a wonderful job going until this entire world generated out of no-where