cmd.script does not work properly with bg=True

Description of Issue/Question

cmd.script in combination with source using salt:// and bg=True does not work as expected.

Setup

master and minion are the same. file_roots contains /srv/salt and there is a script.sh in this directory with the following content:

#!/bin/bash
sleep 300

minion process is run on console with salt-minion -t trace.

Steps to Reproduce Issue

first run the script in foreground (default):

salt MINION cmd.script source=salt://script.sh

minion logs:

[INFO    ] Executing command '/tmp/__salt.tmp.NhvUDT.sh' in directory '/root'

and the sleep command is called:

$ ps -ef | grep sleep
root     32331 32330  0 13:31 ?        00:00:00 sleep 300

this works as designed. the problem arises after bg=True is added to kw-args.

salt MINION cmd.script source=salt://script.sh bg=True

this time minion logs the following error:

[DEBUG   ] In saltenv 'base', looking at rel_path 'script.sh' to resolve 'salt://script.sh'
[DEBUG   ] In saltenv 'base', ** considering ** path '/var/cache/salt/minion/files/base/script.sh' to resolve 'salt://script.sh'
[DEBUG   ] LazyLoaded file.user_to_uid
[INFO    ] Executing command '/tmp/__salt.tmp.7zNkaz.sh' in directory '/root'. Executing command in the background, no output will be logged.
[DEBUG   ] Minion return retry timer set to 5 seconds (randomized)
[INFO    ] Returning information for job: 20181029133404427449
/bin/bash: /tmp/__salt.tmp.7zNkaz.sh: No such file or directory

to me it looks like the script is removed (as part of the post-execution cleanup) prior to being passed to shell. this behaviour has been seen on AIX minions reporting to Linux masters.

Versions Report

Salt Version:
           Salt: 2018.3.3
 
Dependency Versions:
           cffi: 1.11.5
       cherrypy: unknown
       dateutil: 2.7.3
      docker-py: Not Installed
          gitdb: 2.0.5
      gitpython: 2.1.11
          ioflo: Not Installed
         Jinja2: 2.10
        libgit2: 0.27.4
        libnacl: Not Installed
       M2Crypto: 0.30.1
           Mako: 1.0.7
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.19
       pycrypto: 3.6.6
   pycryptodome: Not Installed
         pygit2: 0.27.2
         Python: 2.7.15 (default, Jun 27 2018, 13:05:28)
   python-gnupg: Not Installed
         PyYAML: 3.13
          PyZMQ: 17.1.0
           RAET: Not Installed
          smmap: 2.0.5
        timelib: Not Installed
        Tornado: 5.1.1
            ZMQ: 4.2.5
 
System Versions:
           dist:   
         locale: UTF-8
        machine: x86_64
        release: 4.18.16-arch1-1-ARCH
         system: Linux
        version: Not Installed

Start at the cmd.script entry point and trace how source=salt://script.sh is staged and cleaned up when bg=True. Reproduce with the mentioned script.sh containing sleep 300, then verify that the background command can still execute the staged script after the initial command returns.