Nginx disables static file loading early

Question

I have a Flask application that redirects requests that should receive static files to NGINX via x-accel-redirect. In some cases, these downloads will be disabled until completed. For example, through cURL I would see:

curl http://my_server/some_static_file.tar > temp.tar
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
 77 14.4G   77 11.2G    0     0  55.8M      0  0:04:24  0:03:25  0:00:59 58.9M
curl: (18) transfer closed with 3449105332 bytes remaining to read

This happens more often with very large files (10gb +), but I saw how this happens with smaller files ~ 90mb. Nginx access logs show requests received and served by different, incomplete volumes of data:

1.2.3.4 - - [18/Apr/2017:01:16:26 +0000] "GET /some/flask/static/file/path HTTP/1.1" 200 15146008576 "-" "curl/7.38.0" "5.6.7.8"
1.2.3.5 - - [18/Apr/2017:01:16:29 +0000] "GET /some/flask/static/file/path HTTP/1.1" 200 15441739776 "-" "curl/7.38.0" "6.7.8.9"

errors.log has nothing useful.

My corresponding bulb configuration is as follows:

response = make_response('')
response.headers.set('X-Accel-Redirect', '/_special_nginx_path/' + file_name)
response.headers.set('Content-Disposition', 'attachment',
                     filename=file_name)
# have tried both with and without setting content-length
response.headers.set('Content-Length', os.path.getsize(file_path))
try:
    response.mimetype = mimetypes.guess_type(file_name)[0]
    if not response.mimetype:
        response.mimetype = 'application/octet-stream'
except AttributeError:
    response.mimetype = 'application/octet-stream'
return response

My corresponding NGINX configuration is as follows (where the uWSGI server on which my flask application is running runs at 127.0.0.1:1234):

location / {
            proxy_pass http://127.0.0.1:1234;
            proxy_redirect off;
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }



location /_special_nginx_path {
           internal;
           alias /path/to/static/files;
    }