Cannot return a dict from custom facts

environment.yml

---
- hosts: $host
  tasks:
  - environment:
  - debug: msg="{{ env_HOME }}"
  - debug: msg="{{ environment.HOME }}"
  - debug: msg="{{ environment }}"

library_environment

#!/usr/bin/env python
# this is actually library/environment

import os
import json

env = { 'env_{0}'.format(k): v for k, v in os.environ.items() }

facts = {
    'environment': dict(os.environ),
}

facts.update(env)

print json.dumps({
    'ansible_facts': facts
})

output

$ ansible-playbook -i ../hosts -e host=monitor environment.yml 

PLAY [monitor] **************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [monitor]

TASK: [environment ] ********************************************************** 
ok: [monitor]

TASK: [debug msg="{{env_HOME}}"] ********************************************** 
ok: [monitor] => {"msg": "/root"}

TASK: [debug msg="{{environment.HOME}}"] ************************************** 
ok: [monitor] => {"msg": "{{environment.HOME}}"}

TASK: [debug msg="{{environment}}"] ******************************************* 
ok: [monitor] => {"msg": "{}"}

PLAY RECAP ******************************************************************** 
monitor                    : ok=5    changed=0    unreachable=0    failed=0