Can't group employees list without years

my issue is that when I want to group a set of employees according to the month of their birth date, Odoo separates the months by years, which I want to avoid because i wanna generate a list of people who have their birthday in the month.

This is code:

<filter
    name="birthday_groupby"
    string="Birthday month"
    domain="[]"
    context="{'group_by': 'birthday:week'}"
/>

And this is result:

List screenshot

As you can see, it separates by months, but also by years. I think solution may be more easier than than I think, but i can't figure it out.

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