Custom field - date

Your software
mautic 3.1.1-rc (from git-hub)
php 7.3.22

Your problem
I’ve created a custom field with datatype “Date”. In the Custom Field edit the calender is poping up but not at the contact itself. There is no clue, in which format I have to type in the date. I would like to see the calender pop up at the contact itself, when I click into the birthday field.

Screenshot: Custom Field create / edit page:

Screenshot: The field is not showing a pop-up calender in the contact:
Screenshot 2020-09-22 at 19.55.02

Am I missing something here?