Two squares of land appeared and destroyed what I built.

I created a world in creative mode one year ago, and yesterday I saw two 16x16 squares of land that have cut some house I made. These two blocks seem to restore the landscape and ground level which was there when I created the world and that I edited. I tried to close and relaunch the game and to remove and reinstall the saved world in my PC, but the squares are still here.

Sorry if I made a language mistake or if i wasn't clear enough, English is not my first language.