When a superflat world preset with a stronghold is entered in, Minecraft will save the preset, but without the stronghold.
How to reproduce:
Launch Minecraft, click "Singleplayer", click "Create World" and turn cheats on. Click "More World Options", click "World type - Default" click "Customize", and then click "Presets." Remove all text from the text box and enter
minecraft:bedrock,188*minecraft:stone,2*minecraft:dirt,minecraft:grass_block;minecraft:dark_forest;stronghold
Click "Use Preset", click "Done", and click "Create New World".
Enter command
/locate Stronghold
The message "Could not find that structure nearby" will appear.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-192361, which is a report that has not been resolved yet.
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