Hi all,
after some time of investigation I found the problem. It is caused inside of theFocusElementManager class in Method SetFocusOnLastElement. If I comment out line number 158 the focus problem don't appear anymore. It seems that it only appears if you have some focusable elements in your pane like textboxes or scrollviewers.
if (_modelFocusedElement.GetValue(model, out objectToFocus)) {//focused = objectToFocus == Keyboard.Focus(objectToFocus); }