Create sub-type instances in ASP.NET Core 2.2 - asp.net-core-2.2

Question

I am developing an ASP.NET Core application. There are several entities, some of which have one-to-many relations with each other i.e.
Project 1->* Priority, Project 1->* Milestone; Milestone 1->* Issue...
All navigation properties of all entities are virtual and I have already configured lazy loading in OnConfiguring method in the DbContext class:
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseLazyLoadingProxies();
}
The situation is following: I have already created an instance or Project type and now I want to create instances of other sub-types which should be related to the already created Project instance.
My logic for creating of Priority instance is to pass a projectId to the priority entity which will be added to the corresponding DbSet.
The entity models:
public class Project
{
[Required]
public virtual ICollection<Priority> Priorities { get; set; }
}
public class Priority
{
public int Id { get; set; }
[Required]
public string Name { get; set; }
public string ProjectId { get; set; }
[Required]
public virtual Project Project { get; set; }
}
The priorityInputModel from the form:
public class PriorityInputModel : IMapTo<PriorityServiceModel>
{
[Required]
public string Name { get; set; }
[Required]
public string ProjectId { get; set; }
}
The PriorityServiceModel:
public class PriorityServiceModel : IMapFrom<Priority>, IMapTo<Priority>
{
public int Id { get; set; }
public string Name { get; set; }
public string ProjectId { get; set; }
public ProjectServiceModel Project { get; set; }
}
The controller:
[HttpPost]
public async Task<ActionResult> Create(PriorityInputModel inputModel)
{
var priorityServiceModel = inputModel.To<PriorityServiceModel>();
var priorityServiceModelResult = await this.priorityService.CreateAsync(priorityServiceModel);
return this.RedirectToRoute(
routeName: ValuesConstants.DefaultRouteName,
routeValues: new
{
controller = ValuesConstants.ProjectControllerName,
action = ValuesConstants.DetailsActionName,
id = priorityServiceModelResult.ProjectId,
});
}
The service method:
public async Task<PriorityServiceModel> CreateAsync(PriorityServiceModel priorityServiceModel)
{
var priority = priorityServiceModel.To<Priority>();
var priorityResult = await this.repository.AddAsync(priority);
var priorityServiceModelResult = priorityResult.To<PriorityServiceModel>();
return priorityServiceModelResult;
}
The repository method:
public virtual async Task<TEntity> AddAsync(TEntity entity)
{
await this.DbSet.AddAsync(entity);
await this.SaveChangesAsync();
return entity;
}
I expected when the changes had been saved, the returned Priority object to have initialized Project navigation property with the corresponding project instance but the returned object has value as follows:
Priority {
"Id" = 33,
"Name" = "Lowest",
"ProjectId" = "bd15bd0c-fed7-4186-ade0-ad9338dff1d7"
"Project" = null
}
What is the reason for this and how could I receive the project instance initialized in the navigation property Project of newly created and returned Priority object?
Thanks in advance!