Getting a list of tasks with their custom fields requires you to pass in ?includeCustomFields=true.
A list of tasks will be returned along with the custom fields associated with them. This is a paged API with pagination set at 50. To get the next page, you must pass in page parameter.
If a task doesn’t have any custom fields set, tasks.customFields is omitted from the response, not null.
Please note: The V1 /tasks.json endpoint does not support includeCustomFields=true, you must use this V2 API.