Wasn’t sure if this was intended or a bug but it seems more like an oversight, it would be really nice to have.
Description:
Flat worlds don't have the new height limit (-64 to 320) it only allows you to build from 0 to 256 even though regular worlds have changed.
Thank you for your report!
We're tracking this issue in MCPE-121849, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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