Current XCom __getitem__ behavior is confusing in task mapping context

### Apache Airflow version 2.3.3 (latest released) ### What happened When tasks return dictionary value, it is possible to access the value on the DAG level by using `return_value['key']`. This does not seem to be possible when using the newly added `expand` functionality. It fails to load the DAG with (see example below): `ValueError: cannot map over XCom with custom key 'list' from <Task(_PythonDecoratedOperator): params>` ### What you think should happen instead I think Airflow should behave in a consistent way while using ``` task(return_value['something']) ``` and ``` task.expand(arg=return_value['something']) ``` ### How to reproduce ``` from airflow.decorators import dag, task import pendulum @dag( default_args={ 'owner': 'airflow', 'start_date': pendulum.datetime(2022, 7, 14, tz="Europe/Prague"), 'depends_on_past': False, 'retries': 0, }, schedule_interval="* * * * *", max_active_runs=1, ) def test(): @task(multiple_outputs=True) def params(): return {"a": 1, "b": 1, "list": [1, 2, 3]} parameters = params() @task def print_list(list): return list @task def print_elements(element): return element #this works print_list(parameters['list']) #this also works print_elements.expand(element=[1,2,3]) #this does not print_elements.expand(element=parameters['list']) test_dag = test() ``` ### Operating System Ubuntu 20.0.4 ### Versions of Apache Airflow Providers _No response_ ### Deployment Docker-Compose ### Deployment details _No response_ ### Anything else _No response_ ### Are you willing to submit PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)