pillarenv environments in masterless setup

Description of Issue/Question

I'm using a masterless setup to manage application configuration and deployment on a single machine.

My problem seems similar to #36153 and #30234 , with the difference of a masterless setup, and salt version involved.

I'm trying to use pillar environments to enable multiple deployments of an application with different sets of configurations.

So I have my pillar_roots configured like so:

pillar_roots:
  base:
    - /srv/pillar
  alpha:
    - /srv/pillar/alpha
  beta:
    - /srv/pillar/beta

and my /srv/pillar/top.sls looking like this:

base:                                                                                                                                                                                                                                                                                                                       
  "*":
    - otherstuff

alpha:
  "*":
    - myapp1
    - myapp2
    
beta:
  "*":
    - myapp1
    - myapp2

But testing with e.g. salt-call -l info --local pillar.items myapp1 pillarenv=alpha fails to give the expected results, instead showing the content of beta. I understand it's merging the pillar files in the order they are processed(if I add a key in the alpha version, but not in the beta version, It will be present in the output).

I also tested with pillar_source_merge_strategy: none in my masterless config, after finding out about this, to no effect.

Setup

/etc/salt/minion.d/masterless.conf:

file_client: local

file_roots:
  base:
    - /srv/salt/
  alpha:
    - /srv/salt/alpha
  beta:
    - /srv/salt/beta
                                                                                                                                                                                                                                                                                  
pillar_roots:
  base:
    - /srv/pillar
  alpha:
    - /srv/pillar/alpha
  beta:
    - /srv/pillar/beta

Steps to Reproduce Issue

/srv/pillar/alpha/test.sls:

test:                                                                                                                                                                                                                                                                                                                       
  a: 1
  b: 2
  c: 3

/srv/pillar/beta/test.sls:

test:                                                                                                                                                                                                                                                                                                                       
  a: 3
  b: 4

$ salt-call -l info --local pillar.items test pillarenv=alpha                                                                                                                                                                                                                                     
local:                                                                                                                                                                                                                                                                                                                      
    ----------
    test:
        ----------
        a:
            3
        b:
            4
        c:
            3           

Versions Report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

# salt-call --version                                                                                                                                                                                                                                                                             
salt-call 2019.2.0 (Fluorine)   

# uname -a                                                                                                                                                                                                                                                                                        
Linux server-04 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Start with the masterless configuration in /etc/salt/minion.d/masterless.conf and the salt-call --local pillar.items test pillarenv=alpha entry point. Compare how the alpha and beta pillar_roots and top.sls files are processed, focusing on why beta values appear in the alpha result. Done means the alpha result contains alpha data without beta-only values while preserving the expected merged keys.