That's because in the Austrian and only that versions of the Intrastat export there is:
IF PeriodYear < 20 THEN
Period4 := '20' + Period
Period4 := '19' + Period;
Makes no sense and the fix is obvious (it should be always 20), just saying as the deadline for submitting the January Intrastat is upon us, every customer will get annoyed emails from the Statistics Office and every consultant will be overwhelmed by it.