(I am using the Google translation. Please forgive me if there is any
grammatical mistake. I'm not good at English.)

A gray flag represents a dirt room.As shown, you can find more than 20 dirt rooms on a map with the highest magnification. The abandoned mineshafts surrounding these dirt
rooms cover almost 70 percent of the underground area in this map. You
can find abandoned mines everywhere,they made undergound a mess.

A single abandoned mineshaft covers an area of about 200x200 to
400x400 blocks.They tend to generates together. In some special
seeds,some abandoned mineshafts can cover an area of more than 600x600 blocks.

If you use the /Locate command to Locate an abandoned mineshafts, the
distance between you and the nearest dirt room is never more than 400
blocks.This problem is even more serious in the Superflat world.They
are generated more frequently than dungeons.If you don't believe me,
you can create an save and try the /locate command.

This bug seems to have been around since version 1.13.