Preset I used: 2;1x7,40x42;1;
What I expected to happen:
40 layers of Iron Blocks, 1 Bedrock layer on the bottom and a Plains Biome
What really happened:
Default superflat settings generated in the world
Steps to reproduce:
1) Create a new superflat world in Singleplayer
2) Enter "2;1x7,40x42;1;" into the custom preset
3) Exit the preset selection
4) Hit the "Customize" button under "Superflat" again
You should see that the preset didn't end up staying.
Linked issues
Comments


Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.

Forward resolving to MC-65442, which @unknown has marked "Won't Fix".
OK, I know that this was from a long time ago and that I'm not officially a helper, but I think I have a soloution-
Do the custom preset in 1.12.2, then make the world. Then update the world to whatever version it is when you read this, and then you can play. That's how I fixed mine.
Hope I helped!🙂
Superflat customization hasn't supported numerical block IDs since Minecraft 14w08a. You need to use the block names now:
The game tries to update the old preset string to the current version, but fails at some point. If you put in a version 2 string like
and click "Use Preset", it will load the proper blocks in the list, but if you then click the Presets button again, you'll see that it's updated the version number from 2 to 3, at which point clicking "Use Preset" will revert it back to the default, because block IDs aren't supported in version 3 of the preset format. Clicking "Done" on the Superflat Customization screen probably runs the same check.