Hello Mojang, when upgrading my world to Minecraft version 1.18, some chunks which were loaded but have never been ventured into since 1.17 have reset and is now replaced by an ocean. Luckily, though, I did backup my world.

Reproduce (requires a Windows PC or iPadOS/iOS device)

• Create a world in the Minecraft version 1.17.41 or earlier, but not before 1.17.0.
• Load all chunks within your render distance, preferably wait a couple of seconds and maybe even a minute so that your chunks load properly in all directions.
• Exit the world and load it into version 1.18.0.
• Notice that some chunks which are a little bit further away from you, have not been touched yet still visible, are replaced by 1.18.0 terrain generation.

Expected Results

• Loaded chunks should not reset when upgrading to 1.18.0.