Ansible /etc not writable even after using become

Question

I can't write to /etc with successful privilege escalation using become. I can however write to it directly from the host so sounds like become isn't behaving as expected.

Playbook (I've omitted other tasks for brevity but all tasks in this playbook require escalation to postgres user and are successful)

- name: Playbook Control
  hosts: all
  become: yes
  become_user: postgres

  tasks:
    - name: Debug Perms
      shell: ls -lrt /etc/pgbackrest.conf
    
    - name: Initialize pgbackrest.conf
      copy:
        src: pgbackrest_init
        dest: /etc/pgbackrest.conf

Inventory

hostname.net

[all:vars]
ansible_connection=ssh
ansible_user=pg_deployment

Playbook Command

ansible-playbook -vvv --vault-password-file=.vault_pw -i hosts/hosts playbook_control.yml

Using vault to store pg_deployment ssh password.

Debug Perms Output

"stdout": "-rw-r--r-- 1 postgres postgres 82 Aug 24  2021 /etc/pgbackrest.conf"

Initialize pgbackrest.conf Output

"msg": "Destination /etc not writable"

In the -vvv block preceding the task Initialize pgbackrest.conf, I see escalation succeeded.