Wrong bottom space filling of sub-schematics

[AllayBreeze]

Describe the bug
The sub-schematics use the same bottom as the main schematic for bottom space filling.

Steps to Reproduce
Create a structure that contains sub-schematics, enable BottomSpaceFill, and generate it.

Expected behavior
The bottom space filling of sub-schematics should be proceed based on the own bottom shape.

Screenshots

Structure File(s)
https://pastebin.com/2zfBjTqX

Environment Information:

• Minecraft Version: 1.19.2
• Spigot Version: Purpur-1858
• Custom Structures Version: 1.9.0.1
• World Edit Version: 7.2.14+6360-094b734

Additional context
The air.schem in structure file is a 1 air block schematic.
This structure file is also the one I met issues and mentiond below #164.