[MCM-1697] World generation changes after world has been converted

Partner Rank: 1

Partner Team Reporting: 5 Frame Studios

Verification builds: Found in: 1.20.30 and 1.20.2021. Not found in 1.20.13. Not tested in any other versions. (Windows 11 - but likely not relevant).

Summary

After a world has been converted from Minecraft Java Edition to Minecraft Bedrock Edition (version 1.20.20 or newer), the chunk generation in the converted world exhibits irregularities.

Impact

This likely affects all content that has (a) been converted at least once and (b) is played on version 1.20.30 or newer. For players, this is frustrating as large chunk irregularities are not an ideal experience (shocking really) and for partners, this adds a significant amount of work (and for some contents can't be fixed) to manually fix. We (5 Frame Studios) have fixed this issue by manually blending the original terrain with the "new" terrain using WorldEdit, though this is far from a solution and more of a workaround.

Repro Steps

Step 1: Create a world on Minecraft Bedrock Edition version 1.20.20 or newer.
Step 2: Open the world and save a few chunks. I did this by filling a section of blocks in the chunks around the player, as this forces the game to save those chunks.
Step 3: Export the world.
Step 4: Convert the world to Minecraft Java Edition version 1.20.1 using any available conversion software. This bug has been found with conversion on both Amulet Editor and Chunker, leading me to believe this bug is not related to the conversion software.
Step 5: Load the converted world using Minecraft Java Edition and make some changes to the world.
Step 6: Convert the world back to Minecraft Bedrock edition.
Step 7: Open the converted world with Minecraft Bedrock edition.
Step 8: Go a distance equivalent to your sim distance away from spawn (in my case 5 chunks) - this is because the chunks within this distance are not affected (likely because they were generated by the bedrock engine).
Step 9: Observe issue. Unnatural chunk generation occurs and it almost appears that the engine is using a new seed to generate chunks outside of what was part of the world pre java conversion.

Repro Rate: 100% across several partners.

Observed Results

Unnatural chunk generation occurs and it almost appears that the engine is using a new seed to generate chunks outside of what was part of the world pre java conversion.

Expected Results

Chunk generation to be smooth and not abruptly change.

Videos attached: https://clipchamp.com/watch/qb3nol1LASI

Regression Builds: 1.20.15

Notes: This issue appears to have some similarities with this one, though there are notable differences with their appears to cut off chunks while attempting to blend while this one appears to use a new seed for generation (though the seed is the same throughout the entire process).