Ansible include_vars sets host facts (host vars)

localhost.yml

# host_vars/localhost.yml
---
var1: inventory

test_include_vars.yml

---
- hosts: localhost
  gather_facts: no
  vars:
    var1: play
  tasks:
    - debug:
        var: var1

    - debug:
        var: hostvars[inventory_hostname].var1

    - include_vars: vars.yml

    - debug:
        var: var1

    - debug:
        var: hostvars[inventory_hostname].var1

- hosts: localhost
  gather_facts: no
  tasks:
    - debug:
        var: var1

    - debug:
        var: hostvars[inventory_hostname].var1

vars.yml

---
var1: include_vars