At a module width of 1, every ratio from 2.0 to 2.9 prints as 2:1. The ratio you set is silently quantised away.
Raise the module width, or set the ratio you actually get.
^XA^BY1,2.5^FO50,50^B3N,N,100,Y,N^FD123456^FS^XZ
^BY with module width w = 1 and a ratio between 2.0 and 2.9, on Code 39 or Interleaved 2 of 5.
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.
The bar width is outside the GS1-128 range (0.495 mm to 0.940 mm, 0.495 mm target). At 203 dpi that means ^BY 4 or greater.
Raise ^BY to at least 4 at 203 dpi, or 6 at 300 dpi.
At 203 dpi ^BY2 — the power-up default — and ^BY3 both fail GS1.
At a module width of 1, every ratio from 2.0 to 2.9 prints as 2:1. The ratio you set is silently quantised away.
Raise the module width, or set the ratio you actually get.