Anemone theme has adopted this feature because of the following reasons,
– It is part of modern and distraction free design.
– It is also helpful in saving the space at small viewports like mobiles.
– Menu will re-appear as the user will scroll up.
I will suggest to leave it as default.
I have also posted ticket to our development department. I will share the CSS Code Snippet with you, if it is possible via CSS hack only.