This is how I tried to upload the local build to UCB.

uploading-artifact-chunk-or-no-chunk.py

# perform_bad_gateway == True: how I first called the API and the server ended up crashing
# perform_bad_gateway == False: how I call it NOW and the server has no issue with it

from tusclient import client

upload_url = "https://build-artifact-api.cloud.unity3d.com/api/v1" + \
    "/projects/{0}/buildtargets/{1}/builds/{2}/artifacts/{3}/upload/{4}".format(
        project_id,
        "_local",
        build_number,
        artifact_name,
        artifact_file_name
)

t_client = client.TusClient(upload_url)

t_client.set_headers({"Authorization": "Basic {0}".format(MY_API_KEY), "content-type": "application/offset+octet-stream"})

if perform_bad_gateway:
    # Notice that I set the chunk size to 200. I did this to see the progress.
    # However, If this value is present, the server fail to complete the request.
    uploader = t_client.uploader(file_stream=open("file/to/upload", "rb"), chunk_size=200)
else:
    # If chunk_size is absent, then 64-bit maximum integer is used.
    uploader = t_client.uploader(file_stream=open("file/to/upload", "rb"))
    
while uploader.file_size > uploader.offset:
    uploader.upload_chunk()