How can I keep the master-detail icon in a Navigation page instead of having a back button?

I have read through a lot of threads on here asking similar things, and I have tried all of the fixes suggested, but nothing I do seems to work. I am pretty sure my code is not that different. I have made the master page (navigation drawer) available on every page, but without an icon to say it is there, it is not very useful to users. It is available only with a gesture.

NavigationMenuMasterDetailPage.cs

using MyApp.Localization;
using MyApp.Statics;
using Xamarin.Forms;

namespace MyApp.Pages.Common
{
    public class NavigationMenuMasterDetailPage : MasterDetailPage
    {
        NavigationMenuMasterPage masterPage;
        NavigationPage _navPage = new NavigationPage();

        public NavigationMenuMasterDetailPage()
        {
            masterPage = new NavigationMenuMasterPage();
            Master = masterPage;
            Detail = _navPage;
            _navPage.PushAsync(new StartPage());

            masterPage.MenuListView.ItemSelected += OnItemSelected;
        }

        void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
        {
            var item = e.SelectedItem as MasterPageItem;
            if (item != null && item.Title == TextResources.Settings)
            {
                Detail = _navPage;
                _navPage.PushAsync(new SettingsPage());
            }
            else if (item != null && item.Title == TextResources.Message_Box)
            {
                Detail = _navPage;
                _navPage.PushAsync(new MessageBoxPage());
            }
            else if (item != null && item.Title == TextResources.Login_Log_Out)
            {
                MessagingCenter.Send(this, MessagingServiceConstants.LOGOUT);
            }
            masterPage.MenuListView.SelectedItem = null;
            IsPresented = false;
        }
    }
}

In this page I make the NavigationPage separate so I can also use the back button to go back to the start page, rather than close the app. StartPage displays fine, with the hamburger menu icon as expected.

NavigationMenuMasterPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MyApp.Pages.Common.NavigationMenuMasterPage"
             xmlns:statics="clr-namespace:MyApp.Statics;assembly=MyApp"
             Title="Navigation drawer"
             Icon="hamburger.png">
  <ContentPage.Content>
    <StackLayout VerticalOptions="FillAndExpand">
      <ListView x:Name="menuListView" VerticalOptions="FillAndExpand" SeparatorVisibility="None">
        <ListView.ItemTemplate>
          <DataTemplate>
            <TextCell Text="{Binding Title}" TextColor="{x:Static statics:Palette.Primary}" />
          </DataTemplate>
        </ListView.ItemTemplate>
      </ListView>
    </StackLayout>
  </ContentPage.Content>
</ContentPage>

NavigationMenuMasterPage.xaml.cs

using MyApp.Localization;
using System.Collections.Generic;
using Xamarin.Forms;

namespace MyApp.Pages.Common
{
    public partial class NavigationMenuMasterPage : ContentPage
    {
        public ListView MenuListView { get { return menuListView; } }

        public NavigationMenuMasterPage()
        {
            InitializeComponent();

            var masterPageItems = new List<MasterPageItem>();
            masterPageItems.Add(new MasterPageItem
            {
                Title = TextResources.Settings,
                TargetType = typeof(SettingsPage)
            });
            masterPageItems.Add(new MasterPageItem
            {
                Title = TextResources.Message_Box,
                TargetType = typeof(MessageBoxPage)
            });
            masterPageItems.Add(new MasterPageItem
            {
                Title = TextResources.Login_Log_Out,
            });
            menuListView.ItemsSource = masterPageItems;
        }
    }
}

The problem arises when I go to any page from the Master page, or go further in the flow from the StartPage.

I can post my SettingsPage as an example, but it is extremely basic. All the .cs file does at this point is InitializeComponent();.

SettingsPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="HolteApp.Pages.Common.SettingsPage"
             Icon="hamburger.png">
  <Label Text="This will be the settings page." VerticalOptions="Center" HorizontalOptions="Center" />
</ContentPage>

StartPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MyApp.Pages.StartPage"
             xmlns:l10n="clr-namespace:MyApp.Localization;assembly=MyApp"
             xmlns:statics="clr-namespace:MyApp.Statics;assembly=MyApp"
             Icon="hamburger.png">
  <ContentPage.Content>
    <StackLayout VerticalOptions="FillAndExpand"
                 HorizontalOptions="FillAndExpand"
                 Orientation="Vertical"
                 Spacing="0">
      <Label Text="{l10n:Translate Company_Label}"
             TextColor="{x:Static statics:Palette.Primary}"
             Margin ="16, 16, 16, 8"
             FontAttributes="Bold"/>
      <Label x:Name="CompanyNameView"
             Text="{Binding CompanyName}"
             TextColor="{x:Static statics:Palette.PrimaryText}"
             Margin ="16, 8, 16, 8" >
        <Label.GestureRecognizers>
          <TapGestureRecognizer Tapped="CompanyNameClicked" NumberOfTapsRequired="1" />
        </Label.GestureRecognizers>
      </Label>
      <Label Text="{l10n:Translate Projects_Label}"
             TextColor="{x:Static statics:Palette.Primary}"
             Margin ="16, 8, 16, 8"
             FontAttributes="Bold"/>
      <ListView ItemsSource="{Binding Projects}"
                x:Name="ProjectsView"
                ItemSelected="ProjectClicked">
        <x:Arguments>
          <ListViewCachingStrategy>RecycleElement</ListViewCachingStrategy>
        </x:Arguments>
        <ListView.ItemTemplate>
          <DataTemplate>
            <TextCell Text="{Binding ProjectName}" TextColor="{x:Static statics:Palette.PrimaryText}"/>
          </DataTemplate>
        </ListView.ItemTemplate>
      </ListView>
    </StackLayout>
  </ContentPage.Content>
</ContentPage>

StartPage.xaml.cs

using MyApp.Localization;
using MyApp.Pages.Routines;
using MyApp.ViewModels;
using MyCloudContracts.DTOs;
using System;
using Xamarin.Forms;

namespace MyApp.Pages
{
    public partial class StartPage : ContentPage
    {
        private StartPageViewModel _viewModel;

        public StartPage()
        {
            InitializeComponent();
            Title = TextResources.AppName;
            _viewModel = new StartPageViewModel();
            _viewModel.Init();
            BindingContext = _viewModel;
        }

        private async void ProjectClicked(object sender, SelectedItemChangedEventArgs e)
        {
            //since this is also called when an item is deselected, return if set to null
            if (e.SelectedItem == null)
                return;

            var selectedProject = (CMSProjectInfoDTO) e.SelectedItem;
            var projId = selectedProject.Id;
            var name = selectedProject.ProjectName;
            var handbooksPage = new HandbooksPage(projId, name, false);
            await Navigation.PushAsync(handbooksPage);

            //take away selected background
            ((ListView)sender).SelectedItem = null;
        }

        private async void CompanyNameClicked(object sender, EventArgs e)
        {
            var name = _viewModel.CompanyName;
            var handbooksPage = new HandbooksPage(null, name, true);
            await Navigation.PushAsync(handbooksPage);
        }
    }
}

And then the HandbooksPage comes up, also with the back arrow instead of the master detail icon. I can still access the master page by swiping from the left side of the screen.

Does anybody know what I am doing wrong? I have set the "Icon" attribute on the Master page. I have tried to put my navigation all inside of one Detail page. I really just want to replace the detail page every time, but I need the stack to navigate.