Mineshafts generate twice when updating from 1.17 to 1.18

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.