Java JFrame Removing Components Weirdly by Accident

My Java JFrame project has been buggy with the components, removing everything JComboBox has used in space.

The video example of the bug in my program can be viewed visually here.

Main code is as below. You can replicate it by replacing the 3 Panels with random JCombobox(es) and using the main class to call it.

Any help would be appreciated.

package com.company;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class DisplayPage extends JFrame implements ActionListener {
EnterDetailsPage newRecord;
ViewMore viewMoreRecord;
ViewPage viewRecord;
JPanel contentPanel = new JPanel(new FlowLayout());
public DisplayPage(){
setLayout(new BorderLayout());
setSize(900, 600);
setTitle("Add Flight Details");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setPage(1);
MenuSection menuSection = new MenuSection();
//Add action listener to buttons
menuSection.addNewButton.addActionListener(this::addActionPerformed);
menuSection.updateButton.addActionListener(this::updateActionPerformed);
menuSection.viewButton.addActionListener(this::viewActionPerformed);
add("West", menuSection.getMainMenu());
add("Center", contentPanel);
setVisible(true);
}
public void setPage(int selection){
contentPanel.removeAll();
contentPanel.revalidate();
//contentPanel.repaint();
switch (selection) {
case 1:
newRecord = new EnterDetailsPage();
contentPanel = newRecord.getTotalPanel();
break;
case 2:
viewMoreRecord = new ViewMore();
contentPanel = viewMoreRecord.getViewMorePanel();
break;
case 3:
viewRecord = new ViewPage();
contentPanel = viewRecord.getUpdatePagePanel();
break;
}
add("Center", contentPanel);
}
public void actionPerformed(ActionEvent e){
}
public void addActionPerformed(ActionEvent e){
setPage(1);
}
public void updateActionPerformed(ActionEvent e){
setPage(2);
}
public void viewActionPerformed(ActionEvent e){
setPage(3);
}
}

Instead of bothering making your own GUI, always look through the java documentation in order to check if there is anything similar to what you want to use.
Instead of making your own "MenuSection", there already are JTabbedPanes, which you can use much more easily (without having flickering and other graphical issues).
https://docs.oracle.com/javase/tutorial/uiswing/components/tabbedpane.html

Alternatively, you could use CardLayout.
https://docs.oracle.com/javase/tutorial/uiswing/layout/card.html

Example for CardLayout:

public class DisplayPage extends JFrame implements ActionListener {
 ...

    CardLayout cl;
 ...

    public DisplayPage(){
     ...
        
        cl = new CardLayout();
        contentPanel.setLayout(cl);
     ...

        contentPanel.add(newRecord,"newRecord");
        contentPanel.add(viewMoreRecord,"viewMoreRecord");
        contentPanel.add(viewRecord,"viewRecord");
     ...
        cl.show(contentPanel,"newRecord");
     }


    public void addActionPerformed(ActionEvent e){
        cl.show(contentPanel,"newRecord");
    }

    public void updateActionPerformed(ActionEvent e){
        cl.show(contentPanel,"viewMoreRecord");
    }

    public void viewActionPerformed(ActionEvent e){
        cl.show(contentPanel,"viewRecord");
    }