If you create a map under Superflat Tunneler's Dream under one name in creative with the bonush chest option on, then delete it, then create a normal survival map under the same name regardless if the the bonus chest is on or not, chunks from the supperflat world appear.
Watch the video I made to get a better understanding of the issue, and then try it yourselves.
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Thanks i didn't know how it worked.
42nd Duplicate of MC-315, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.