When prefWidth of the label decreases it clips the text, though it must use ellipsis.

To reproduce shrink window width and there will be no ellipsis.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.OverrunStyle;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Label_ellipsis extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception
    {
        Label label = new Label("The quick brown fox jumps over the lazy dog 01234567890");
        label.setEllipsisString("...");
        label.setTextOverrun(OverrunStyle.ELLIPSIS);

        VBox root = new VBox(10d);
        root.getChildren().add(label);
        
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

}