But the field 1_9 is not displayed if the the date format in field 1_3 is dd.mm.yy and not dd.mm.yyyyy.
So my first Idea was to check if the date format is valid and notificate the user.
Alternatively I wondered to switch off the input option and only use the date picker. That would be fine for me as well. While using the date picker it works as expected.