item.sync() sends invalid data to servers

[ghost]

I'm not sure if this is an issue with the API itself or if I just have a wrong understanding, however, I found out that I can send invalid data to the server using api.sync() and that those data are accepted by the server and additionally saved and fetched on the next synchronisation.
To make it easier to understand I wrote a code snippet explaining what I mean:

api = todoist.TodoistAPI(token)
api.sync()

for item in api.state["items"]:
    if item["content"] == "test #issue65" and isinstance(item["due"], dict): # makes sure that we only modify an item that the content "test #issue65" and a due-date
        print(item["due"])
        item["due"] = ["i can put here", "whatever i want"]
        print(item["due"])
        api.sync()
        print(item["due"])

The result I get is the following:

{'date': '2019-10-29', 'timezone': None, 'is_recurring': False, 'string': 'Okt 29', 'lang': 'de'}
['i can put here', 'whatever i want']
['i can put here', 'whatever i want']

Creating a new Todoist-API-instance doesn't change anything:

def run_through():
    print("running through ...")
    token = "token"
    api = todoist.TodoistAPI(token)
    api.sync()
    for item in api.state["items"]:
        if item["content"] == "test #issue47":
            print(item["due"])
            if isinstance(item["due"], dict):  # makes sure that we only modify an item that the content "test #issue47" and a due-date
                item["due"] = ["i can put here", "whatever i want"]
                print(item["due"])
                api.sync()
                print(item["due"])

run_through()
run_through()

running through ...
{'date': '2019-10-29', 'timezone': None, 'is_recurring': False, 'string': '29 Okt', 'lang': 'de'}
['i can put here', 'whatever i want']
['i can put here', 'whatever i want']
running through ...
['i can put here', 'whatever i want']