After updating to 1.2.13 superflat worlds I've converted from Minecraft: XboxOne Edition stop generating terrain, and instead any new chunks either have air straight down to the void (replicated) or generate "tile.info_update.name" in place of the expected blocks (which only seems to happen to worlds made before the Better Together Update was released. See screenshot).
What I expected: Chunks generated within the newer engine continue to follow the custom settings of the superflat world. (I'd even accept settings being reset to that of a default superflat world.)
What happened: Chunks generated within the newer engine (and specifically after this update) are filled with air and nothing else (blocks can still be placed within these new chunks) or in worlds made before before the BTU, the settings are completely
replaced, with bedrock being swapped for air, and any blocks other than bedrock or air being swapped for "tile.info_update.name" (which can be mined, picked up, and placed).
How to replicate:
1. Create a superflat world in Minecraft: XboxOne Edition.
2. Use the "Sync Old Worlds" option in Minecraft to convert this world.
3(method one). Enter this converted world in Minecraft, save and quit, then enter the world again.
3(method two). In the world select menu of Minecraft, enter the "Edit "[world name here]" Game Settings" menu for this converted world, then enter the world.
4. Travel to chunks that have not yet been generated in Minecraft: XboxOne Edition or older versions of Minecraft.
5. Notice the broken terrain generation.