I made a superflat world made of only oak planks in the plains biome. Then I found this underground
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Duplicate of MC-65628.
Thank you for your report!
However, this issue is Invalid.
Your Minecraft version is outdated. We currently take issues for version 1.17.1 and the latest snapshot.
Please update to the latest version as it includes the newest fixes. If you still have this problem after updating, then please create a new issue.
In case of a game crash, please also attach the crash report found in [minecraft/crash-reports/crash-<DATE>-client.txt|https://minecrafthopper.net/help/finding-minecraft-data-folder/].
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Your screenshot shows 1.17 Release Candidate 1, this is an outdated (and thus, unsupported) version. If you can reproduce the same issue on the latest version, please also attach the seed, coordinates and the exact template used for superflat world you generated.