Could you try pushing already formed page instead of forming new one every time and check if this fixes the problem? Like to change it to:

public partial class Sidebar : FlyoutPage
{
private ApplicationSettings AppSettingsPage;
    public Sidebar()
    {
        InitializeComponent();
        Detail = new NavigationPage(new MainPage());
        AppSettingsPage = new ApplicationSettings();
    }

 private async void GoToApplicationSettingsPage(object sender, TappedEventArgs e)
 {
    
     await Detail.Navigation.PushAsync(AppSettingsPage);
     IsPresented = false;
 }

}

Or try to change the behaviour from PushAsync to PushModalAsync, like

await Shell.Current.PushModalAsync(new ApplicationSettings());