gistfile1.txt

test_dir
├── roles
│   ├── a
│   │   ├── tasks
│   │   │   ├── subtasks
│   │   │   │   ├── main.yml
│   │   │   │   │   - debug: msg="{{ lookup('file', 'test_relative.yml') }}"
│   │   │   │   ├── test_relative.yml
│   │   │   │   │   - debug: msg="works before 2.16.3"
│   │   │   ├── main.yml
│   │   │   │
├── playbook.yml
│   - hosts: localhost
│     gather_facts: no
│     tasks:
│       - include_role:
│           name: a
│           tasks_from: subtasks/main.yml

Solution 1: tasks/main.yml can be removed to to restore the original non-role search path

Solution 2: use the path recognized within the context of the role according to https://docs.ansible.com/ansible/latest/playbook_guide/playbook_pathing.html#resolving-local-relative-paths (in this example, since it's looking up a 'file' and it isn't in the files directory, it should be "{{ lookup('file', 'tasks/subtasks/test_relative.yml') }}"