This is something that happens on a testing world originally created on 1.17 but updated to the newest Pre-release. The mineshaft is generated on Y 21, but by updating it also generates a mineshaft at Y-27 that looks exactly the same. I can't test with locate which one is "real" because that command does not tell the Y value.
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We're tracking this issue in MC-240494, so this ticket is being resolved and linked as a duplicate.
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I looked around for more to find, and saw another mineshaft generated twice. So it can't be a coincidence.