counter number format not used in tax invoice

[Paul de Vries]

gnucash 2.6.18 on mac osx

i want the current year as part of the invoice number, so in book options i specified the invoice number format as 'F2018-%04d'. 
on the tax invoice the the invoice number is shown as '000001'.

changing the format using %ld %li %lld %lli doesn't change the output so i suspect the definition is elsewhere.

a workaround would be to use a format like '2018001', numeric, with counter at '2018001', but i want the option for more variations.

how can i do this?





searching through the sources i found:
./Contents/Resources/share/gnucash/scm/gnucash/report/taxinvoice.eguile.scm: (invoiceid (gncInvoiceGetID opt-invoice))
./libgnucash/engine/gncInvoice.c:
const char * gncInvoiceGetID (const GncInvoice *invoice) { if (!invoice) return NULL; return invoice->id; }
but i cannot find where this struct is built. is the printable form stored in the database or created at print time?