ADD 1 can usually be used to well... add 1, but once you reach 0x7FFF, ADD 1 no longer works and just gets stuck.

This is a problem for programs that use and manipulate large numbers. Why this occurs? No idea, sorry.

Version: tis3d-MC1.20.1-forge-1.7.7+354a583.jar
Loader: NeoForge 1.20.1 47.1.104

Look, I know it's unsupported in this version, but usually Forge mods work fine under NeoForge.

This isn't an issue, 0x7FFF corresponds to the highest value and 0x8000 is the lowest negative value. The behavior of not overflowing is questionable, but it is what it is.