Automapper ignores the mapping for derived class

Question

I have a class hierarchy as described below:

 public partial class EmployeeDTO
    {
        public virtual long Id { get; set; }

        public virtual string Name { get; set; }

        public virtual string ReportsTo { get; set; }
    }

  public class ManagerDTO: EmployeeDTO
    {
        public override string ReportsTo
        {
            get => null;
            set => throw new InvalidOperationException("ReportsTo can not be set to other than the preset value.");
        }
    }

   public class EmployeeModel
    {
        public long Id { get; set; }

        public string FullName { get; set; }

        public EmployeeType Type { get; set; }

        public string ReportsTo { get; set; }
    }

   public enum EmployeeType
    {
        Employee = 0,
        Manager = 1,
    }

Mapping configuration

           CreateMap<EmployeeDTO, EmployeeModel>()
                .ForMember(empmodel => empmodel.FullName, empdto => empdto.MapFrom(empdto => empdto.Name))
                .ForMember(empmodel => empmodel.Type, empdto => empdto.MapFrom(empdto => EmployeeType.Employee));

            CreateMap<ManagerDTO, EmployeeModel>()
               .ForMember(model => model.FullName, dto => dto.MapFrom(dto => dto.Name))
               .ForMember(model => model.Type, dto => dto.MapFrom(dto => EmployeeType.Manager));

            CreateMap<EmployeeModel, ManagerDTO>()
               .ForMember(dto => dto.Name, model => model.MapFrom(model => model.FullName))
               .ForMember(t => t.ReportsTo, t => t.Ignore());

            CreateMap<EmployeeModel, EmployeeDTO>()
                .ForMember(dto => dto.Name, model => model.MapFrom(model => model.FullName));

I am trying to patch the entity as follows:

       [HttpPatch]
       public IActionResult Patch([FromBody] Delta<EmployeeDTO> entityDelta)
       {
           var existingModel = new EmployeeModel
           {
               Id = 123,
               FullName = "XYZ",
               Type = Shared.EmployeeType.Manager,
               ReportsTo = null
           };

           var existingEntity = _mapper.Map<EmployeeDTO>(existingModel);
           entityDelta.Patch(existingEntity);

           // Following mapping incorrectly sets type of updatedModel as EmployeeType.Employee
           var updatedModel = _mapper.Map(existingEntity, existingModel);
           return Ok(updatedModel);
       }

After executing the above code expected type of the updatedModel to be Manager, but the type is Employee instead. I have tried using the other overloads for the Map. Map(entity) works as expected when explicit type is provided for the previous mapping ie. Map(model). However, this overload creates another instance of the dbmodel with the same id, which leads to further issues. To avoid having two runtime DbEntries with the same id, I am trying to use the Map(entity, existimgModel) overload of the mapper, which is failing to give the desired outcome. Any ideas on how to solve this?

Sample code