In new world generation there can be floating mineshafts. In 1.12.2 this doesn't occur.
The exact reasons are unknown and this cannot be reproduced reliably, see the comments of this report.
/tp @p -623 82 993 200 40
Mesa mineshafts are broken, cut off at chunk borders
Mesa mineshaft spawning outside of caves.
Mesa surface mineshafts generate wood in the air
Mesa surface mineshafts place cave_air even if mineshaft piece did not place blocks
Mesa mineshafts often generate inefficiently
Mesa mineshafts are on top of the ground AND in another biome