Village structures do not replace the bottom layer of the world, even if it isn't bedrock

The bug

In superflat worlds, villages will not attempt to replace blocks at the minimum y level with blocks that are part of their structure.

How to reproduce

Create two worlds, one using the first preset and the other using the other preset, and compare how villages look:

1. minecraft:glass;minecraft:plains

2. 2*minecraft:glass;minecraft:plains

Expected results

The glass would be replaced by paths, house floors, and so on, since it isn't bedrock or any other block that should not be replaceable.

Actual results

It is not replaced.