existing superflat world has big holes after update

I have this world for 4 years, yesterday I worked on more buildings before finding out about huge holes in my world (didn't affect any buildings).
I copied a backup world from a month ago and everything is normal the first time I play it. I exit to menu and rejoin, holes of same locations appear, and it absolutely ruined my world.

after: https://imgur.com/a/kCF4G

before: https://imgur.com/a/Un0Uq

my beautiful world below