Use include_vars in sub playbook from parent playbook

I have a playbook playbook.yml, which import sub playbooks, and I want to include play2.yml conditionally based on my_app_vars[key1]

- import_playbook: play1.yml
- import_playbook: play2.yml
  when: {{ my_app_vars[key1] }}
  vars:
    key1: "{{mock_key}}"

in play1.yml, I have

- name: "Get Some Info"
  hosts: mock_servers
  pre_tasks:
    - name:
      include_vars:
        file: mock-vars.yml
        name: my_app_vars
      register: "{{my_app_vars}}"  #adding register did not help

After execution, I get the error my_app_vars is undefined, after adding register, still get the same error.

I could use my_app_vars, mock_key and key1 under a role correctly in play2.yml.

But now I want to use my_app_vars, mock_key and key1 in playbook.yml, is there any way to achieve it?
I use ansible-playbook-2.8.1, thanks in advance

Update:

follow answer from @β.εηοιτ.βε, I tried below [no register in play1.yml]

when: not ( {{ hostvars[groups[mock_servers][0]].my_app_vars[key1] }} )
''mock_servers'' is undefined'

when: not ( {{ hostvars[groups['mock_servers'][0]].my_app_vars[key1] }} )
''ansible.vars.hostvars.HostVarsVars object'' has no attribute ''my_app_vars'''

when: not ( {{ hostvars[groups.mock_servers.0].my_app_vars[key1] }} )
''ansible.vars.hostvars.HostVarsVars object'' has no attribute ''my_app_vars'''

when: not ( {{ hostvars[groups.mock_servers.0] }}.my_app_vars[key1] )
''dict object'' has no attribute ''my_app_vars'

After add gather_facts: false, work as expected, fantastic

This is implicitly stated in the last bullet point of the module synopsis:

> * To assign included variables to a different host than inventory_hostname, use delegate_to and set delegate_facts=yes.

<sup>Source: https://docs.ansible.com/ansible/latest/collections/ansible/builtin/include_vars_module.html#synopsis&lt;/sup&gt;

Which means the variables included by include_vars are actual stored as hosts variables.

This means, that, in order to access them in your main playbook, that includes the two other playbooks and which is not selecting any hosts or groups, you will have to access those via the hotsvars special variable.

In your case it could be something like:

when: hostvars[groups.mock_servers.0].my_app_vars[key1]

An example of that behaviour would be as follow.

play.yml:

- import_playbook: play1.yml
  
- import_playbook: play2.yml
  when: hostvars[groups.group1.0].vars_of_play1.foo == 'bar'

play1.yml:

- hosts: group1
  gather_facts: false

  pre_tasks:
    - include_vars:
        file: vars.yml
        name: vars_of_play1

vars.yml:

foo: bar

play2.yml:

- hosts: group2
  gather_facts: false

  pre_tasks:
    - debug:

Which would, as expected, run the second playbook:

$ ansible-playbook play.yml 

PLAY [group1] ************************************************************

TASK [include_vars] ******************************************************
ok: [ansible-node-1]

PLAY [group2] ************************************************************

TASK [debug] *************************************************************
ok: [ansible-node-2] => 
  msg: Hello world!