This report is a more detailed report of someone who had a very similar issue to me: 

Link Here: https://bugs.mojang.com/browse/MC-252552?jql=project%20%3D%20MC%20AND%20text%20~%20%221.19%20world%22%20ORDER%20BY%20updated%20DESC 

  

  

This issue occurred when I opened an older world in 1.19. 

  

One world was a 1.17 super flat world with generating structures and various gamerules. When I loaded up the world a large chunk of biome generated below me surrounded by a mote of superflat world, soon to continue into the Overworld. It was like a normal Minecraft world with an inverted square doughnut in the middle. 

When I first created the world, it was in version 1.18.2. But later on, I opened it in 1.17 Minecraft Forge, changing the version to 1.17. Recently I opened the world in 1.19 to try out the update, which caused the terrain issue. 

No mods or texture packs where at use during the bug discovery.  

 

  

The other world that experienced the bug was downloaded for testing my texture packs, initially the world was 1.18.2, but then I accidentally opened the world in Minecraft Forge 1.17 (Therefore changing the version).   

When I loaded up the world in 1.19, a mesa / ocean biome generated in the void removing all blocks and mobs (except the ones summoned).  

I obtained the world from: https://www.planetminecraft.com/project/restworld-a-complete-resourcepack-testing-world-for-1-15-2/ 

A texture pack was at use during the bug discovery. 

 

 

 

 

 

{*}Re-creating the bug. (I was unable to exactly re-create the bug)
{*}The mods I had didn't use custom biome or structure generation. 

 

Vanilla Minecraft 1.17.1 to 1.19. (Bug didn't happen)

1. I opened Minecraft and created a superflat world in 1.17.1.
2. I reopened Minecraft in 1.19, the game updated successfully.

Vanilla Minecraft 1.19 to 1.17.1. (Game generated a new world)

1. I opened Minecraft and created a superflat world in 1.19.
2. I reopened Minecraft in 1.17, the game created a new world. (Minecraft did warn me)

Minecraft Forge 1.17.1 to 1.19. (Bug didn't happen)

1. I opened Minecraft and created a superflat world in Forge 1.17.1.
2. I reopened Minecraft in 1.19, the game updated successfully.

Minecraft Forge 1.19 to Vanilla 1.19. (Bug didn't happen)

1. I opened Minecraft and entered a 1.19 superflat world in Forge 1.19.
2. I reopened Minecraft in 1.19, the game updated successfully.

Vanilla 1.18.2 to Forge 1.17.1 to Vanilla 1.19. 

(The same thing happened as 1.19 to 1.17.1)

Vanilla 1.19 to vanilla 1.18.2.

(Nothing happened)

 

 

 

The report creator for this website is also buggy.