Broken jinja2 include inside loop

base_specific1.j2

specific1

output for test-context.j2

ubuntu@2378491cf3ce:/template-bug$ ansible-playbook test.yml -e test_template=test-context.j2
 [WARNING]: Host file not found: /etc/ansible/hosts

 [WARNING]: provided hosts list is empty, only localhost is available


PLAY [localhost] ******************************************************************************************************************************************************************

TASK [Test template] **************************************************************************************************************************************************************
fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "msg": "KeyError: 'undefined variable: 0'"}
	to retry, use: --limit @/template-bug/test.retry

PLAY RECAP ************************************************************************************************************************************************************************
localhost                  : ok=0    changed=0    unreachable=0    failed=1

output for test-nocontext.j2

ubuntu@2378491cf3ce:/template-bug$ ansible-playbook test.yml -e test_template=test-nocontext.j2
 [WARNING]: Host file not found: /etc/ansible/hosts

 [WARNING]: provided hosts list is empty, only localhost is available


PLAY [localhost] ******************************************************************************************************************************************************************

TASK [Test template] **************************************************************************************************************************************************************
fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "msg": "AttributeError: 'NoneType' object has no attribute 'add_locals'"}
	to retry, use: --limit @/template-bug/test.retry

PLAY RECAP ************************************************************************************************************************************************************************
localhost                  : ok=0    changed=0    unreachable=0    failed=1

test-context.j2

{% set b = file_base %}
{% for p in file_part %}
{% include file_base ~ p ~ '.j2' %}
{% endfor %}

test-nocontext.j2

{% set b = file_base %}
{% for p in file_part %}
{% include file_base ~ p ~ '.j2' without context %}
{% endfor %}

test.yml

# Test playbook
# .
# ├── templates
# │   ├── base_specific1.j2
# │   ├── test-context.j2
# │   └── test-nocontext.j2
# └── test.yml

- hosts: localhost
  gather_facts: no
  vars:
    test_template: test-context.j2
    file_base: 'base_'
    file_part:
      - specific1
  tasks:
    - name: Test template
      template:
        src: "{{ test_template }}"
        dest: "{{ playbook_dir }}/output-rendered.txt"