it's a bug and has nothing to do with flagging. Test it, if you log out of your character and try to log in (fast) with another one, you will not be able to. I am sure there are other reasons too, but I have had 100's of times where I was sitting in my house for 2+ hours doing absolutely nothing (wanted to play, but couldn't get up the ambition for the travesty which was pvp), and would log out and then into another character, and have to wait 5 minutes. In my house. No spawn for screens.