Python Threading at random returns wrong results

My Problem

I have been using the python library Thread, and it seems to have some issues returning the correct results. When I run the same function e.g. ten times in a row, eight times the results are correct and two times they are wrong.

When the results are wrong, it is because some of the results-dictionarys from the individual calls have been seemingly randomly merged together.

My Code:

This function makes a session that retries rest calls for certain status codes:

# Makes retry sessions
def requests_retry_session(retries=3,backoff_factor=0.3,status_forcelist=(500, 502, 504),session=None):
    """
    Description:
        Creates a session which uses retries
    Input:
        retries (int):  Max number of retries
        backoff_factor (int): Time between retries
        status_forcelist (tuble): Status for which to retry
    Returns:
        session: Requests session which handles different status and connection errors
    """
    session = session or requests.Session()
    retry = Retry(
        total=retries,
        read=retries,
        connect=retries,
        redirect=retries,
        backoff_factor=backoff_factor,
        status_forcelist=status_forcelist,
    )
    adapter = HTTPAdapter(max_retries=retry)
    session.mount('http://', adapter)
    session.mount('https://', adapter)
    return session

This function makes rest calls for multiple urls:

def make_rest_calls(urls, header, store=None):
    """
    Description:
        Processes list of urls
    Input:
        urls (list): List of urls for rest call
        header (dictionary): Dictionary containing credentials
        store (dictionary): Dictionary for collecting results
    Returns:
        store (dictionary): Dictionary with results
    """
    if store is None:
        store = {}
    for url in urls:
        store[url] = requests_retry_session().get(url, headers=header, timeout=5)

    return store

This function runs rest calls multithreaded

def run_multi_threaded(nthreads, list_of_urls, header):
    """
    Description:
        Runs multiple threads
    Input:
        nthreads (int): Number for threads to run
        list_of_urls(list): List of rest urls
        header (dictionary): Dictionary containing credentials
    Returns:
        store (dictionary): Dictionary with results
    """
    store = {}
    threads = []

    # create the threads
    for i in range(nthreads):
        small_list_of_urls = list_of_urls[i::nthreads]
        t = Thread(target=make_rest_calls, args=(small_list_of_urls))
        threads.append(t)

    # start the threads
    [t.start() for t in threads ]
    # wait for the threads to finish
    [ t.join() for t in threads ]

    return store

Questions

Is this a weakness of the package? Should I use multiple processes instead? Or am I doing something wrong that results in this side effect only some times?

I need to make MANY calls, so it needs to be done multithreaded. The obviously also has to be correct.

from Python Threading at random returns wrong results