var person = new Employee
{
    Office = new Office(),
    Foo = new Dummy()
};

person.Office.People = [person];

var options = new JsonSerializerOptions { ReferenceHandler = ReferenceHandler.IgnoreCycles };
var json = JsonSerializer.Serialize(person, options);
Console.WriteLine(json);

[JsonDerivedType(typeof(Employee), "Employee")]
abstract class Person
{
}

class Employee : Person
{
    // public Dummy Foo { get; set; } Issue does not occur here

    public Office Office { get; set; }

    public Dummy Foo { get; set; } // Issue occurs here
}

class Office
{
    // Doesnt matter what type of collection is being used, IEnumerable/List/Array/Dictionary
    public List<Person> People { get; set; }
    // public Dummy Foo { get; set; } Issue also occurs here
}

class Dummy
{
}