The mineshaft support pillars, or the logs that support the mineshaft when it's in the open air, are unable to generate on foliage blocks which makes them float.

Steps to recreate:
1. Create a single biome world with the lush caves

2. Search for a mineshaft

3. Keep searching until you find a support beam being blocked from generating by a foliage block

World I used:

Single Biome world, Lush Caves

Generation Options: Default (Structures on, bonus chest off, etc.)

Seed:1240668140915194828

Coordinates: X -10, Y -10, Z -95

For 21w13a:

Single Biome world, Lush Caves

Generation Options: Default (Structures on, bonus chest off, etc.)

Seed:1240668140915194828

Coordinates: X -207, Y -44, Z -1501