Occasionally, mineshafts are cut off at chunk borders. However, this can not be consistently reproduced; sometimes the generated parts don't generate at all.

To reproduce

Example 1

Seed: -5237585497757630354

Coordinates: 907 6 681
This one block wide part of the mineshaft will sometimes generate, and sometimes it won't.

Example 2

Seed: -7729344434077138248

Coordinates: 217 -50 -471
This spawner will sometimes generate, and sometimes it won't.

Coordinates: 196 -51 -468
This part of the mineshaft will sometimes generate, and sometimes it won't.

(more examples, see screenshots)