[BUG] Pillars are empty when running through custom execution module with salt-ssh

Description
I'm trying to access pillars inside a custom execution module using salt-ssh, but the __pillars__ dict is empty.

Using __salt__['pillar.get']() though.

Setup
I'm executing via a docker container in my local machine, targetting a Digital Ocean droplet.

# Dockerfile
ARG SALT_VERSION
FROM saltstack/salt:${SALT_VERSION}
ARG SALT_VERSION
RUN apk add --no-cache openssh-client
RUN USE_STATIC_REQUIREMENTS=1 pip3 install --no-cache-dir salt-ssh=="${SALT_VERSION}" pyopenssl
CMD ["salt-ssh"]

# composer.yaml
services:
  ssh:
    build:
      args:
        SALT_VERSION: 3004
    hostname: salt
    volumes:
      - ./etc/master:/etc/salt/master:ro
      - ./etc/roster:/etc/salt/roster:ro
      - ./roots:/srv/salt:ro
      - ./Saltfile:/root/.salt/Saltfile:ro
      - $HOME/.ssh/id_rsa:/etc/salt/pki/master/ssh/salt-ssh.rsa:ro

# /etc/salt/master

file_client: local

file_roots:
  base:
    - /srv/salt/base/salt
    - /srv/salt/formulas/mysql-formula
    - /srv/salt/formulas/users-formula
    - /srv/salt/formulas/redis-formula
    - /srv/salt/formulas/docker-formula

pillar_roots:
  base:
    - /srv/salt/base/pillar

# /etc/salt/roster
minion:
    host: my.example.net
    set_path: '/usr/local/opt/pyenv/versions/3.7.12/bin:$PATH'  # to make sure I have the same version on both sides

Here's /srv/salt/ structure:

.
├── base
│   ├── pillar
│   │   ├── ...
│   │   └── top.sls
│   └── salt
│       ├── _modules
│       │   └── helpers.py
│       ├── ...
│       └── top.sls
└── formulas
    ├── docker-formula
    ├── mysql-formula
    ├── redis-formula
    └── users-formula

# pillar/init.sls

user: myuser

# salt/_modules/helpers.py

def pillar_get(key, default=None, **options):
    return __salt__['pillar.get'](key.replace('/', ':'), default)

# to debug
def pillars():
    return ', '.join(__pillar__.keys())

Steps to Reproduce the behavior

$ docker compose run --rm ssh salt-ssh minion pillar.get user
minion:
    myuser
$ docker compose run --rm ssh salt-ssh minion helpers.pillar_get user
minion:
    None
$ docker compose run --rm ssh salt-ssh minion helpers.pillars
minion:

Expected behavior
I expect that __pillar__ inside _modules/helper.py is filled, and that __salt__['pillar.get']() works.

As you can see, it works when executing salt-ssh minion pillar.get directly. Also, salt-ssh minion pillar.items returns my pillars, so it's being matched correctly in pillar/top.sls.

Versions Report

Salt Version:
          Salt: 3004
 
Dependency Versions:
          cffi: 1.14.6
      cherrypy: 18.6.1
      dateutil: 2.8.1
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 2.11.3
       libgit2: 1.1.0
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: 2.17
      pycrypto: Not Installed
  pycryptodome: 3.9.8
        pygit2: 1.6.1
        Python: 3.7.12 (default, Sep  8 2021, 01:55:52)
  python-gnupg: 0.4.4
        PyYAML: 5.4.1
         PyZMQ: 18.0.1
         smmap: Not Installed
       timelib: 0.2.4
       Tornado: 4.5.3
           ZMQ: 4.3.1
 
System Versions:
          dist: alpine 3.14.2 
        locale: UTF-8
       machine: x86_64
       release: 5.10.76-linuxkit
        system: Linux
       version: Alpine Linux 3.14.2

minion:
    Salt Version:
              Salt: 3004
     
    Dependency Versions:
              cffi: Not Installed
          cherrypy: Not Installed
          dateutil: Not Installed
         docker-py: Not Installed
             gitdb: Not Installed
         gitpython: Not Installed
            Jinja2: 2.11.3
           libgit2: Not Installed
          M2Crypto: Not Installed
              Mako: Not Installed
           msgpack: 1.0.2
      msgpack-pure: Not Installed
      mysql-python: Not Installed
         pycparser: Not Installed
          pycrypto: Not Installed
      pycryptodome: Not Installed
            pygit2: Not Installed
            Python: 3.7.12 (default, Feb 10 2022, 23:34:36)
      python-gnupg: Not Installed
            PyYAML: 5.4.1
             PyZMQ: Not Installed
             smmap: Not Installed
           timelib: Not Installed
           Tornado: 4.5.3
               ZMQ: Not Installed
     
    System Versions:
              dist: ubuntu 16.04 xenial
            locale: UTF-8
           machine: x86_64
           release: 4.4.0-148-generic
            system: Linux
           version: Ubuntu 16.04 xenial

Additional context
I've been using salt-master for a number of years, but I'm trying now to depend only on SSH. So my configs have been working for a long time, I'm just trying to make it all work with salt-ssh.

I've shutdown my master and the minion service to depend only on SSH.

Begin with the salt-ssh execution of custom modules and compare it with direct pillar.get; inspect salt/_modules/helpers.py and the supplied master and pillar configuration. Reproduce the three commands, then verify that pillar is populated and helpers.pillar_get returns the pillar value.