ADDITIONAL SYSTEM INFORMATION :
javafx-sdk-11.0.2 - reproducible on latest LTS
javafx-sdk-14.0.1 - reproducible on latest Release
javafx-sdk-15 - reproducible on latest EA
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openjdk 14.0.2 2020-07-14
OpenJDK Runtime Environment (build 14.0.2+11-45)
OpenJDK 64-Bit Server VM (build 14.0.2+11-45, mixed mode, sharing)
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Operating System: Ubuntu 20.04 LTS
                  Kernel: Linux 5.4.0-40-generic
         Architecture: x86-64

A DESCRIPTION OF THE PROBLEM :
From VBox API doc: VBox lays out its children in a single vertical column.
And It does so. But being aligned with 'Pos.BASELINE_CENTER' VBox allocates additional top-margin for another nested VBox. Height of that margin seems to be the same with that nested VBox's.
You can find screenshots in related question from StackOverFlow: https://stackoverflow.com/questions/62842756/javafx-how-to-correctly-put-one-vbox-inside-another


---------- BEGIN SOURCE ----------
        VBox childVBox = new VBox(50.0);
        childVBox.setBackground(new Background(new BackgroundFill(Color.GREEN, null, null)));
        childVBox.getChildren().add(new Label("hell1"));
        childVBox.getChildren().add(new Label("hell2"));
        childVBox.getChildren().add(new Label("hell3"));

        VBox parentVBox = new VBox(20.0);
        parentVBox.setAlignment(Pos.BASELINE_CENTER);
        parentVBox.setBackground(new Background(new BackgroundFill(Color.ORANGE, null, null)));
        parentVBox.getChildren().add(new Label("JAVAFX v." + System.getProperties().get("javafx.runtime.version")));
        parentVBox.getChildren().add(new Separator());
        parentVBox.getChildren().add(childVBox);

        Scene scene = new Scene(parentVBox, 400, 400);
        primaryStage.setTitle("VBox Bug");
        primaryStage.setScene(scene);
        primaryStage.show();
---------- END SOURCE ----------

FREQUENCY : always