There is a mistake (or change) in framework default value special vars.

In manual :

There are several values that you can use here that will be replaced automatically when a new record is created:

    <<MM>> - the current month, 2 digits
    <<DD>> - the current day of month, 2 digits
    <<YEAR>> - the current year, 4 digits
    <<USER>> - the username of the currently logged in user

For example: a default of "<<MM>>/<<DD>>/<<YEAR>>" (without quotes) will print the current date in the form of "01/21/2021"

See
https://gitlab.com/koha-community/koha-manual/-/blob/master/source/administration.rst

But in Koha code in cataloguing/addbiblio.pl :
        $value =~ s/<<YYYY>>/$year/g;
        $value =~ s/<<YY>>/$shortyear/g;