Reported by clicking on Avast Passwords icon on google chrome, then clicking on "Report a problem" and sending a message text with the problem explained.
Yes, of course I did.
Settings to ask the password:
1)Password has been set;
2)How often should Avast ask for the password: twice a day;
3)Lock the password vault after closing internet browser: not marked.
Yes, as (probably) 95% of Windows 10 users.
From my observations it seems, that "twice a day" really means twice a day - Avast does not care that you have turned off your laptop 8 hours ago and now (maybe) someone else is using the computer: the user (whoever it is) will be asked to enter the password only every 12 hours.
This means that the note in the password settings "Note: Avast will ask for your Main Password every time you restart your computer" is misleading.
Now I have marked "lock the password vault after closing internet browser" and that seems to work.