missing chunks (partial)

Hi!
I got a problem! I lost 1 chunk after re-entering the world! Only those blocks that I put myself have disappeared. The blocks that the world created (super flat world) are not gone.
Red line-(in the picture) blocks that created by the world itself. I have a strong computer and I have no lags!
I have not used third-party mods on this version (Forge, optfine ...)