When updating an old world to add layers below previous bedrock, new above ground terrain overrides previous terrain as well as removes entire chunks from the world in random locations.
Related issues
Attachments
Comments

Has the world been edited in the world editor?
Is it possible to upload the world affected here? If it's too big you can upload it elsewhere and link it here.

This report has been temporarily closed as Awaiting Response. It will automatically reopen when you comment with new information. Please be sure to provide all requested information. Reports that do not contain enough information may be closed as Incomplete.
You may find this Report Creation Form helpful for any future reports you create.

The world has not been edited in world editor, but it was imported from IOS, as trying to update it via IOS kept saying “not enough storage available” when there is plenty. How would one go about uploading a world here via Windows?
In the world settings, next to the delete button, there will be an export button.

Where would be a good place to upload the world and link it? Seems to be too large to link it directly.

JAMES, upload it to a cloud service. I personally use Google Drive or OneDrive (both under my game name) for my upload need.

https://drive.google.com/file/d/1j3zlSEhofCjXffe4sgeADDk_ZndofjlV/view?usp=sharing
Let me know if it doesn't work. What are you looking for here?
An empty file is downloaded from your link with the name: New Buildh.mcworld
😞

Are you wanting the updated or non updated world? Because it appears the non updated version will not import back into Minecraft. Could this be an issue with the fact that is was imported from Pocket Edition?

It would be whichever you encountered the bug in. Also, if you made the .mcworld file from the main world folder, it will break. Select the contents of the folder then zip it into the .mcworld file.

https://drive.google.com/drive/folders/1-Gi17Dks-mfOq1Qbztt0E4KQyVZg0tR0?usp=sharing
https://drive.google.com/file/d/1tlyCB6BeKBwxoNdXyk4rUyi-PgCiPOg4/view?usp=sharing
Hopefully one of these should work. Thank you for being patient while I figure this out.

Thank you for your report!
We're tracking this issue in MCPE-150757, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
-- I am a bot. This action was performed automatically! The ticket was resolved by one of our moderators, and I left this message to give more information to you.
@unknown
I downloaded the world by the link https://drive.google.com/file/d/1tlycb6bekbwxondxyk4ruyi-pgcipog4/view and imported it. What are the cords of the place where the chunks disappeared? I can not find damaged chunks.