Structure Blocks Fail to Save at Negative Y

Version Information

Minecraft version: 1.12.2
Forge version: 14.23.5.2860
CC version: 0.0.1257.0

Steps to reproduce:

1. Create a 'structure' at a negative Y level.
2. Attempt to save the structure using structure blocks.
3. The structure will not save, but instead an error will be produced stating nothing can be found.

Expected Behavior: Structures should save at negative Y levels.

Actual Behavior: Structures do not recognize corner blocks or save while Y is beneath 0.

##Additional Context
I have not tested functionality at Y levels exceeding 255, but the error may occur in other contexts as well.