Serialization doesn't seem to maintain the structure

defaults.yaml

# -*- coding: utf-8 -*-
# vim: ft=yaml
---
borg:
  backup_key: '/root/.ssh/backup_key'
  borgmatic:
    location:
      source_directories:
        - /etc
        - /home
    retention:
        prefix: "{fqdn}-"
        keep_minutely: 60
        keep_hourly: 48
        keep_daily: 7
        keep_weekly: 5
        keep_monthly: 6
        keep_yearly: 1
    consistency:
        prefix: "{fqdn}-"
    exclude_patterns:
        - '*.pyc'
        - '*.swp'
        - '~/*/.cache'
        - '/var/cron/log*'

init.sls

{% from "borg/map.jinja" import borg with context %}
backup ssh key:
  cmd.run:
    - name: 'ssh-keygen -q -t ed25519 -C "{{ salt.grains.get('id') }} backup key" -f {{ borg.backup_key}} -N ""'
    - creates: {{ borg.backup_key }}
  module.wait:
    - mine.send:
      - ssh_backup_key
      - mine_function: ssh.user.keys
      - user: root
      - pubfile: '{{borg.backup_key}}.pub'
      - prvfile: False
    - watch:
      - cmd: backup ssh key
borg backup programs:
  pkg.installed:
    - pkgs:
      - {{borg.borg_package}}
      - {{borg.borgmatic_package}}

borgmatic configuration test:
  file.serialize:
    - name: /tmp/bm-test.yaml
    - formatter: yaml
    - dataset: {{borg.borgmatic}}

map.jinja

# -*- coding: utf-8 -*-
# vim: ft=jinja
{#- Get the `tplroot` from `tpldir` #}
{%- set tplroot = tpldir.split('/')[0] %}
{#- Start imports as #}
{%- import_yaml tplroot ~ "/defaults.yaml" as default_settings %}
{%- import_yaml tplroot ~ "/osarchmap.yaml" as osarchmap %}
{%- import_yaml tplroot ~ "/osfamilymap.yaml" as osfamilymap %}

{#- Retrieve the config dict only once #}
{%- set _config = salt['config.get'](tplroot, default={}) %}

{%- set defaults = salt['grains.filter_by'](
      default_settings,
      default=tplroot,
      merge=salt['grains.filter_by'](
        _config,
        default='lookup'
      )
    )
%}

{%- set defaults = salt['grains.filter_by'](
      default_settings,
      default=tplroot,
      merge=salt['grains.filter_by'](
        osarchmap,
        grain='osarch',
        merge=salt['grains.filter_by'](
          osfamilymap,
          grain='os_family',
          merge=salt['grains.filter_by'](
            _config,
            default='lookup'
          )
        )
      )
    )
%}

{%- set config = salt['grains.filter_by'](
      {'defaults': defaults},
      default='defaults',
      merge=_config
    )
%}

{% set borg = config %}

osarchmap.yaml

# -*- coding: utf-8 -*-
# vim: ft=yaml
#
# Setup variables using grains['osarch'] based logic.
# You just need to add the key:values for an `osarch` that differ
# from `defaults.yaml`.
# Only add an `osarch` which is/will be supported by the formula.
#
# If you do not need to provide defaults via the `osarch` grain,
# you will need to provide at least an empty dict in this file, e.g.
# osarch: {}
---
amd64:
  arch: amd64

x86_64:
  arch: amd64

osfamilymap.yaml

# -*- coding: utf-8 -*-
# vim: ft=yaml
#
# Setup variables using grains['os_family'] based logic.
# You just need to add the key:values for an `os_family` that differ
# from `defaults.yaml` + `osarch.yaml`.
# Only add an `os_family` which is/will be supported by the formula.
#
# If you do not need to provide defaults via the `os_family` grain,
# you will need to provide at least an empty dict in this file, e.g.
# osfamilymap: {}
---
FreeBSD:
  repo_base: '/export/backup/borg'
  zfs_base: 'backups/borg'
  user_shell: '/bin/sh'
  borg_package: 'py37-borgbackup'
  borgmatic_package: 'py37-borgmatic'
Arch:
  repo_base: '/var/lib/borg/repos'
  user_shell: '/bin/bash'
  borg_package: 'borg'
  borgmatic_package: 'borgmatic'
OpenBSD:
  borg_package: 'borgbackup'
  borgmatic_package: 'borgmatic'
  borgmatic:
    location:
      source_directories:
          - /etc
          - /home
          - /var/backups
          - /var/db/pkg
          - /var/unbound
          - /var/nsd

resulting bm-test.yaml

consistency:
  prefix: '{fqdn}-'
exclude_patterns:
- '*.pyc'
- '*.swp'
- ~/*/.cache
- /var/cron/log*
location:
  source_directories:
  - /etc
  - /home
  - /var/backups
  - /var/db/pkg
  - /var/unbound
  - /var/nsd
retention:
  keep_daily: 7
  keep_hourly: 48
  keep_minutely: 60
  keep_monthly: 6
  keep_weekly: 5
  keep_yearly: 1
  prefix: '{fqdn}-'