I see your Point. The Input is technically invalid.
But it's the datepicker that actually changes/ messes up the user input by doing this semi- validation step of converting a 4 digit Year into a 2 digit Year.
My suggestion would be to do it right, or dont do it at all. Either completly validate the field with datepicker and convert the year sensibly.
Or don't do it at all and pass along the data as entered. - That way i could at least set up proper validation/ handling for this on the backend.
As for the magical guesswork:
I don't think there is guessing involved to fix the 4 Digit to 2 Digit conversion. When somone enters 4 Digits in a place where a Year is expected, we can safely assume that we need to look at the last two digits when we want to save it in a 2 digit Year format. - I can't think of any exceptions here.
Using maxLength to work around this would work i guess. But in a CMS Setting where datepicker is used in some core component this is not that easy fixable.