This happens in go_format_output_to_odf.

We do not recognize the format family and so use:

	default: {
		/* We need to output something and we don't need any details for this */
		int date = 0, digit = 0;
		char const *str = go_format_as_XL (fmt);
		while (*str != '\0') {
			switch (*str) {
			case 'd': case 'm': case 'y': case 'h': case 's':
				date++;
				break;
			case '#': case '.': case '0': case 'e':
				digit++;
				break;
			default:
				break;
			}
			str++;
		}
		if (digit < date)
			go_format_output_date_to_odf (xout, act_fmt, name,
						      GO_FORMAT_DATE, with_extension);
		else
			go_format_output_general_to_odf (xout, name, cond_part);
		result = FALSE;
		break;

Which effectively outputs a default format string.

This problem has been fixed in the development version. The fix will be available in the next major software release. Thank you for your bug report.