I see your issue. You aren’t making a change to a list of TaskLists…you’re only making a change to a single TaskList. You are updating a list within that single item, but in English, what you are currently saying is this:

“Find me all the TaskLists that exist and add this subtask to every one of them”

What you want to say is this:
“Get me the single, specific TaskList I want to update, and add this subtask to it.”

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