I just had a closer look and yes that's definitely the problem. Fonts require a 1:1 mapping between characters and outputs, there's no way to choose an output based on 2 consecutive digits so it needs to be mapped to an arbitrary character. If you scanned the resulting barcode I'm sure you'd see that it works just fine.
Good point, but that's probably too much work for a free font. Especially since you need to output a code to mode switch before you can use the 2-digit barcodes, the font won't have any way of knowing what mode you're in.
