Chunks of superflat worlds created before 1.18 have portions under "y=0" removed

Random chunk lines in my superflat world (made before 1.18) have been removed due to this update. I am not sure as to why but this has affected all of my superflat world- experimental and not experimental. 

I have provided the world below, as well as pictures if they dont work.

 

If you need more info, let me know. Thank you