...If a character has an estate in Region A, it should be quite impossible to appoint them as Lord of Region B.
If a character has an estate in Region A that is part of Duchy B, appointing them as Duke or Duchess of Duchy B should not pose a problem; however, they will lose their estate come the next morning if they are not also Lord of Region A.
So...I'm not sure what specific problem you're having that isn't a bug.
I think what's been referred to is that when you're elected as a lord you vacate your old estate but don't automatically take up one in the region you're now lord of - and you're forced to wait a day to choose a new estate after having left your old one.