Bug 321755 EPS adds a part without considering its placeholder if no target container or part stacks could be found
The summary probably makes no sense. Will deliver a fix with a test in a bit.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/PartServiceImpl.java
return part;
		case VISIBLE:
			MPart activePart = getActivePart();
			if (activePart == null) {
				bringToTop(part);
			} else if (activePart.getParent() == part.getParent()) {
				
				part.setToBeRendered(true);
				if (part.getCurSharedRef() != null) {
					part.getCurSharedRef().setToBeRendered(true);
					engine.createGui(part.getCurSharedRef());
				} else {
					engine.createGui(part);
				}
			} else {
				bringToTop(part);
			}
			return part;
		case CREATE:
			part.setToBeRendered(true);
			if (part.getCurSharedRef() != null) {
				part.getCurSharedRef().setToBeRendered(true);
				engine.createGui(part.getCurSharedRef());
			} else {
				engine.createGui(part);
			}
			return part;
		}
		return part;
	}