JavaFX split pane divider position not working

Question

I have a split pane with a initial divider position set to 0.2, the problem is that after running the application the position is changed to 0.5, I read that it is due to stage.setMaximized(true);
I checked this question, but the answers imply that the current split pane and the main stage are in the same class, but in my case, I have to work in a controller class.

I tried to add a listener to the stage showing property Main.getStage().showingProperty().addListener(...) in the initialize() method, but it doesn't work, it says stage is null, I guess it's because initialize() runs before Main.start()

So how can I make my split pane start with a fixed divider position?

Thanks!

EDIT :

I made a simple example to expose my problem :

Main

@Override
public void start(Stage primaryStage) throws Exception {
  FXMLLoader loader=new FXMLLoader(getClass().getResource("views/test.fxml"));
  mainStage=primaryStage;
  mainStage.setScene(new Scene(loader.load()));
  mainStage.show();
  Controller_Test ct=loader.getController();
  System.out.println("Main:"+ct.sp.getDividerPositions()[0]);
  ct.getDividerPos();
}

Controller_Test

@FXML
public SplitPane sp;

@FXML
public void initialize() {
    System.out.println("Initialize:"+sp.getDividerPositions()[0]);
}

public void getDividerPos(){
    System.out.println("Method:"+sp.getDividerPositions()[0]);
}

Now I set the split pane divider at 1, which means the second pane should not show, I run the application and here is the result :

Okay for this one, the divider is at 1, so only one pane is showing, now I will maximize the window :

The divider position clearly changed. If you read the code I provide previously, you can see 3 print lines as checkpoints, here is their output after running the application :

Initialize:1.0
Main:0.9933110367892977
Method:0.9933110367892977

Thanks!