iex(6)> :httpc.request(:post, {'http://httpbin.org/post', [], 'application/json', 'any body at all'}, [], [])
{:ok,
 {{'HTTP/1.1', 200, 'OK'},
  [
    {'connection', 'keep-alive'},
    {'date', 'Wed, 05 Sep 2018 05:38:54 GMT'},
    {'via', '1.1 vegur'},
    {'server', 'gunicorn/19.9.0'},
    {'content-length', '308'},
    {'content-type', 'application/json'},
    {'access-control-allow-origin', '*'},
    {'access-control-allow-credentials', 'true'}
  ],
  '{\n  "args": {}, \n  "data": "any body at all", \n  "files": {}, \n  "form": {}, \n  "headers": {\n    "Connection": "close", \n    "Content-Length": "15", \n    "Content-Type": "application/json", \n    "Host": "httpbin.org"\n  }, \n  "json": null, \n  "origin": "159.65.119.183", \n  "url": "http://httpbin.org/post"\n}\n'}}

works fine for me. You should probably check your flask app.