In the following code (I'm using the same code to demonstrate three related bugs) I request setting of dividerPositions directly via SplitPane.setDividerPositions().

If You remove the workaround (single line marked: "// fixed by forcing a relayout!!!"), you will see the divider positions do not move from their default position (roughly 50% divide).
You can force position actualization (only for the vertical divider!) by mouse clicking on a divider.
(actually, if you have a more complex design, partial layout() can be started by any action requiring a redraw, like hovering over a sensitive button or TitlePane).

Note: I did not find a way to attach a file to Issue, so I put the code inline here.
=========================================================
import java.util.Timer;
import java.util.TimerTask;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.geometry.Orientation;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.SplitPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Main2 extends Application {
public static void main(String[] args) {
launch(args);
}

@Override
public void start(Stage primaryStage) throws Exception {
final SplitPane root = new SplitPane();
root.setOrientation(Orientation.VERTICAL);
final SplitPane child;
{
{
child = new SplitPane();
child.setDividerPositions(0.3);
{
HBox h = new HBox();
h.getChildren().add(new Label("LEFT"));
child.getItems().add(h);
}
{
HBox h = new HBox();
h.getChildren().add(new Label("RIGHT"));
child.getItems().add(h);
}
root.getItems().add(child);
}
{
HBox bottom = new HBox();
bottom.getChildren().add(new Label("Bottom"));
root.getItems().add(bottom);
}
}
root.setDividerPositions(0.8);
Scene s = new Scene(root);
primaryStage.setScene(s);
primaryStage.setMaximized(true);

Timer timer = new Timer(true);
timer.schedule(new TimerTask() {
@Override
public void run() {
Platform.runLater(new Runnable() {
@Override
public void run() {
child.setDividerPositions(0.3);
child.layout(); // fixed by forcing a relayout!!!
root.setDividerPositions(0.8);
}
});
}
}, 1000);
primaryStage.show();
}
}
=========================================================