Sometimes, the terrain will create huge stone walls in deep caves under y0. So far I've seen 3 cases, and all of them had something in common. I believe it happens when a dripstone caves biome generates under a badlands biome.
In the screenshot, you can see an example of one of the seeds and coordinates.
Thank you for your report!
We're tracking this issue in MCPE-141342, 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.