no type of new generation is born below layer 0 (mine shafts, lush caves, amethyst, diorite pockets, granite or something similar). neither are entities generated.
Also, no entrances are generated to the caves in the upper part of the world.
All this is activating the experimental options.
Thank you for your report!
We're tracking this issue in MCPE-121708, MCPE-121724, and MCPE-122056, 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 tickets 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