Its not a bug all the time.... I have very often seen this "bug" appear with a perfectly natural explanation to it.
The mouse pointer was touching the side of the screen as you entered the new map, hence scrolling you our of the area where you have been before, into a completely black area, the char is still standing perfectly safe whereever he started, but you cant see it....
Double clicking on the Hero's pic (top right) centres you to your char again.
I think most of the time, this is the "bug".... only once did i actually end up in a black area, and that was in the demo... the reason for me ending up in black: ME getting behind a door that i wasnt supposed to be able to get past, and entering a area that wasnt in the demo.