I came back to my normal superflat, and saw that walls of default randomely grew.It was not there before that day.

Put the summary of the bug you're having here
i came out of my house on superflat, and saw the world was carved out of default.
What I expected to happen was...:
Describe what you thought should happen here
To have an actual superflat world and landscape.
What actually happened was...:
Describe what happened here
Like skyblock, looked like carved out of default.
Steps to Reproduce:
1.I have 0% idea.
2.AND IT IS NOT LIKE MC-315 CUZ IT WAS NOT THERE BEFORE!!!!!!
3. ...