Xamarin application not navigating back - xamarin.forms

Question

Problem
This is the process:
Select a category from list.
Load Tasks page.
Tasks are loaded depending on the categoryId selected from the previous page. (Navigate back to Category page is possible ✔️)
Select a Task from from list.
Load Task Page.
Task details are loaded on the page. (Navigate back to Tasks page is not possible ❌)
Video
Question
I do not understand why I cannot navigate back a page. How can I fix this?
Code
CategoriesViewModel
public class CategoriesViewModel : BaseViewModel
{
public ObservableCollection<CategoryModel> Categories { get; } = new ObservableCollection<CategoryModel>();
public Command LoadCategoriesCommand { get; }
public Command<CategoryModel> SelectedCategory { get; }
public CategoriesViewModel()
{
Title = "Categories";
LoadCategoriesCommand = new Command(async () => await LoadCategories());
SelectedCategory = new Command<CategoryModel>(OnSelectedCategory);
}
private async Task LoadCategories()
{
IsBusy = true;
try
{
Categories.Clear();
var categories = await DatabaseService.GetCategoriesAsync();
foreach (var category in categories)
{
this.Categories.Add(category);
}
}
catch (Exception)
{
throw;
}
finally
{
IsBusy = false;
}
}
private async void OnSelectedCategory(CategoryModel category)
{
if (category == null)
return;
await Shell.Current.GoToAsync($"{nameof(TasksPage)}?{nameof(TasksViewModel.CategoryId)}={category.CategoryId}");
}
public void OnAppearing()
{
IsBusy = true;
}
}
TasksViewModel
[QueryProperty(nameof(CategoryId), nameof(CategoryId))]
public class TasksViewModel : BaseViewModel
{
public ObservableCollection<TaskModel> Tasks { get; } = new ObservableCollection<TaskModel>();
private int categoryId;
public int CategoryId
{
get { return categoryId; }
set
{
categoryId = value;
}
}
public Command LoadTasksCommand { get; set; }
public Command<TaskModel> SelectedTask { get; set; }
public TasksViewModel()
{
Title = "Tasks";
LoadTasksCommand = new Command(async () => await LoadTasks());
SelectedTask = new Command<TaskModel>(OnSelectedTask);
}
private async Task LoadTasks()
{
IsBusy = true;
try
{
this.Tasks.Clear();
var tasks = await DatabaseService.GetTasksAsync(CategoryId);
foreach (var task in tasks)
{
this.Tasks.Add(task);
}
}
catch (Exception)
{
throw;
}
finally
{
IsBusy = false;
}
}
private async void OnSelectedTask(TaskModel task)
{
if (task == null)
return;
await Shell.Current.GoToAsync($"{nameof(TaskPage)}?{nameof(TaskViewModel.TaskId)}={task.TaskId}");
}
public void OnAppearing()
{
IsBusy = true;
}
}
TaskViewModel
[QueryProperty(nameof(TaskId), nameof(TaskId))]
public class TaskViewModel : BaseViewModel
{
private int taskId;
public int TaskId
{
get { return taskId; }
set
{
taskId = value;
}
}
private string taskTitle;
public string TaskTitle
{
get { return taskTitle; }
set
{
taskTitle = value;
OnPropertyChanged(nameof(TaskTitle));
}
}
private string description;
public string Description
{
get { return description; }
set
{
description = value;
OnPropertyChanged(nameof(Description));
}
}
public Command LoadTaskCommand { get; }
public TaskViewModel()
{
LoadTaskCommand = new Command(async () => await LoadTask());
}
private async Task LoadTask()
{
IsBusy = true;
try
{
var task = await DatabaseService.GetTaskAsync(TaskId);
this.TaskTitle = task.Title;
this.Description = task.Description;
}
catch (Exception)
{
throw;
}
finally
{
IsBusy = false;
}
}
public void OnAppearing()
{
IsBusy = true;
}
}
Update 1
I tried replacing this line of code in TasksViewModel:
await Shell.Current.GoToAsync($"{nameof(TaskPage)}?{nameof(TaskViewModel.TaskId)}={task.TaskId}");
to this:
await Shell.Current.Navigation.PushAsync(new AboutPage());
Also, the same outcome.
Update 2
As per requested comment, here is the TaskPage.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"
xmlns:vm="clr-namespace:SomeProject.ViewModels"
x:Class="SomeProject.Views.Task.TaskPage"
Title="{Binding TaskTitle}">
<ContentPage.Content>
<RefreshView x:DataType="vm:TaskViewModel"
Command="{Binding LoadTaskCommand}"
IsRefreshing="{Binding IsBusy, Mode=TwoWay}">
<StackLayout>
<Label Text="{Binding Description}" />
</StackLayout>
</RefreshView>
</ContentPage.Content>
</ContentPage>
and TaskPage.xaml.cs:
public partial class TaskPage : ContentPage
{
TaskViewModel _viewModel;
public TaskPage()
{
InitializeComponent();
BindingContext = _viewModel = new TaskViewModel();
}
protected override void OnAppearing()
{
base.OnAppearing();
_viewModel.OnAppearing();
}
}
Update 3
As per requested comment, here is the routes:
public AppShell()
{
InitializeComponent();
Routing.RegisterRoute(nameof(CategoriesView), typeof(CategoriesView));
Routing.RegisterRoute(nameof(TasksPage), typeof(TasksPage));
Routing.RegisterRoute(nameof(TaskPage), typeof(TaskPage));
}

Answer 1

Check your registers route.
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/shell/navigation#register-page-routes
In the Shell subclass constructor, or any other location that runs
before a route is invoked, additional routes can be explicitly
registered for any pages that aren't represented in the Shell visual
hierarchy

Answer 2

I had CategoryPage registered in AppShell.xaml.cs and also AppShell.xaml like so:
<ShellContent Route="CategoryPage" ContentTemplate="{DataTemplate local:CategoryPage}" />
Only can register one route in one or the other.