Return a polymorphic list from WebAPI

I'm building a small simple REST service using WebAPI in .NET 6.0

I have a base class, Client, with a couple of properties. Then I have three classes that inherit from it, so let's call them TypeAClient, TypeBClient, and TypeCClient. Each have a couple of properties that are only relevant to that type of client.

I have a couple of endpoints

api/customers/{customerId}/clients/{clientId}

api/customers/{customerId}/clients

The first retrieves a single Client record and the second retrieves a list of all Clients for a specific customer.

When I call the first one (returns a single client), it always shows the right properties for the type of client, i.e. if it's a type A client, it shows the properties inherited from the base class and the properties specific to type A.

But if I call the second one, I get a list of Client objects, but each object only contains the properties that are inherited from the base class.

Why are the two methods behaving differently in this respect, and is there a way to get it to include all the properties in the list?

Cut-down version of my controller class below.

Thanks!

  [Route("api/customers/{customerId}/clients")]
  [Authorize]
  [ApiController]

  public class ClientsController : ControllerBase {

    [HttpGet]
    public ActionResult<IEnumerable<Client>> GetClients(int customerId) {
      // Retrieve data
      return Ok(clients);
    }

    [HttpGet("{clientId}", Name = "GetClient")]
    public ActionResult<Client> GetClient(int customerId, int clientId) {
      // Retrieve data
      return Ok(client);
    }
  }

Found an answer that works, although I'm not sure if it's the correct way to do it, or a fudge.

  [Route("api/customers/{customerId}/clients")]
  [Authorize]
  [ApiController]

  public class ClientsController : ControllerBase {

    [HttpGet]
    public ActionResult<IEnumerable<Client>> GetClients(int customerId) {
      // Retrieve data
      List<object> objects = new List<object>(clients);
      return Ok(objects);
    }

    [HttpGet("{clientId}", Name = "GetClient")]
    public ActionResult<Client> GetClient(int customerId, int clientId) {
      // Retrieve data
      return Ok(client);
    }
  }

If I convert my list of Client objects to a list of object objects, then the serialiser calls GetType on each individual element in the list, and the output includes the properties that are specific to the individual subclasses.