I've been struggling with the same issue and I think I found a solution to this that seems to be working consistently (haven't tested this more than 2 minutes, but it seems fine). Found it here https://www.javaprogrammingforums.com/awt-java-swing/4183-swing-components-invisible-startup.html

But basically: I create the JFrame as part of the class attributes (JFrame visibility set false) - then set the components visibility after adding them to the JFrame - then I set the Visibility of the JFrame to true after everything, or in a different method ---

class Main {
public static JFrame Login;

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

public static void Create_Frames() {
    // === Login page GUI creation ===
    Login = new JFrame("Login");
    Login.setSize(400, 200);
    Login.setLayout(null);
    Login.setVisible(false);
    Login.setResizable(false);
    Login.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

    JLabel Name_label = new JLabel("Enter your username:");
    Name_label.setBounds(10, 10, 130, 20);
    //....Other code....
    
    Login.add(Name_label);
    Login.add(Name_input);
    Login.add(Pass_label);
    Login.add(Pass_input);
    Login.add(Login_but);
    Login.add(Register_but);
    Name_label.setVisible(true);
    Pass_label.setVisible(true);
    Name_input.setVisible(true);
    Pass_input.setVisible(true);
    Login_but.setVisible(true);
    Register_but.setVisible(true);
}
// Other method to set frame to true
public static void Login() {
    Login.setVisible(true);
}

If this worked for you please let me know