When attempting to load a structure with a structure block at any y level below 0; the structure will load with the lowest part at y=0 as though 0 was still the lower build limit.
Steps to Reproduce:
- Create new world
- Cheats turned on, Caves and cliffs turned on
- /setblock 0 0 0 structure block
Inside the block menu any Y offset less than zero is reset to zero.
Y offset should go as low as -64 from that level.