This ^FD is not closed with ^FS, so the field definition is never ended and the command that follows is absorbed into it.
End every ^FD with ^FS.
^XA^FO50,50^FDHELLO^FO50,100^FDWORLD^FS^XZ
A ^FD (or ^FV) is not terminated by ^FS before the next field-defining command (^FO, ^FT, ^FB, ^A, ^B*, ^GB, ^XZ).
Neither ^PW nor ^LL is set. Both fall back to a value stored in the individual printer, not to a value in the spec — so the same ZPL prints differently on two printers of the same model.
Set ^PW and ^LL explicitly in every format.
This ^FD is not closed with ^FS, so the field definition is never ended and the command that follows is absorbed into it.
End every ^FD with ^FS.