|Recently I have noticed that even if I go back immediately I have to log in again with my password.|
What do you mean by "go back"?
If you mean you log out of the management interface, well, then you've logged out and going back in, even immediately, requires you to login again. Just for the record, when you break the connection to the management interface by killing the web browser, it's the same thing as logging out. The router knows the connection has been severed because the socket closes.
If you want to leave it open and accessible for a period of time, then you have to either open another tab in your browser, or open another browers entirely, leaving the one with the management interface open.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.