Bug 339573 Splitting the shared area with a part doesn't remove stale placeholders
The current code only looks for the addition of an MPlaceholder in the shared area as the hint to remove locals. When you split the area the element that gets added is the new MPartSashContainer...
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/ModelServiceImpl.java
MWindow containerWin = (MWindow) container;
					if (containerWin.getSharedElements().contains(curElement))
						return IN_SHARED_AREA;
					EObject containerParent = container.eContainer();
					if (containerParent instanceof MPerspective) {
						MElementContainer<MUIElement> perspectiveParent = ((MPerspective) containerParent)
								.getParent();
						if (perspectiveParent == null)
							return NOT_IN_UI;
						int location = IN_ANY_PERSPECTIVE;
						if (perspectiveParent.getSelectedElement() == containerParent)
							location |= IN_ACTIVE_PERSPECTIVE;
						return location;
					} else if (containerParent instanceof MWindow)
						return OUTSIDE_PERSPECTIVE;
					else
						return NOT_IN_UI;
				}
			}
			curElement = parent;
		}
		return NOT_IN_UI;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * org.eclipse.e4.ui.workbench.modeling.EModelService#removeLocalPlaceholders(org.eclipse.e4
	 * .ui.model.application.ui.basic.MWindow,
	 * org.eclipse.e4.ui.model.application.ui.advanced.MPerspective)