The bug

Mineshafts can generate floating (without support) when the surrounding cave generation cannot support logs or chains.

This can occur when the area below the mineshaft is lava and the area above the mineshaft is too large for chains to be able to connect to the blocks above.

How to reproduce

Seed: 6678253584516239758
Coordinates: /execute in minecraft:overworld run tp @s 465 -37 403 0 0

Observed behavior

The mineshaft is generated unsupported because the area below is lava and the blocks above it are too far above.

Expected behavior

The mineshaft would still generate supports of some type, or the mineshaft would not generate in areas like this.