salt: Failure with quickly cascaded salt-ssh state.apply calls to the same minion

Description of Issue/Question

When executing two concurrent state.applyes targeting the same SSH minion and with a small delay in between, one of the states is not applied and it returns an empty response.

The queue=true doesn’t seem to make a difference.

However, without the small delay in between the calls both seem to work fine.

Setup

/srv/salt/echo1.sls

echo1:
  cmd.run:
    - name: 'echo "echo1"'

/tmp/file1:
  file.managed:
    - source: salt://file1.txt

/srv/salt/echo2.sls

echo1:
  cmd.run:
    - name: 'echo "echo2"'

/tmp/file2:
  file.managed:
    - source: salt://file2.txt

/srv/salt/file1.txt

111111111

/srv/salt/file2.txt

2222222222

/etc/salt/roster2

test-minssh-sles12sp3.tf.local:
    host: test-minssh-sles12sp3.tf.local
    user: root
    port: 22
    timeout: 180

test.sh

#!/bin/bash

MINION=test-minssh-sles12sp3.tf.local
KEY=/path/to/sshkey/or/salt/user

cd /srv/salt

sudo -u salt salt-ssh -linfo --roster-file=/etc/salt/roster2 --priv=$KEY $MINION state.apply echo1 queue=true >echo1.log 2>&1 &
sleep 0.5
sudo -u salt salt-ssh -linfo --roster-file=/etc/salt/roster2 --priv=$KEY $MINION state.apply echo2 queue=true >echo2.log 2>&1 &

tail -f --retry /srv/salt/echo1.log /srv/salt/echo2.log

Steps to Reproduce Issue

Make sure salt-ssh can call the target minion (ssh key is authorized, etc)

Execute test.sh. One of the two calls should return an empty response and the corresponding /tmp file is not created on the minion.

Comment the line with sleep 0.5. Both calls should succeed.

Versions Report

Salt Version:
           Salt: 2018.3.0
 
Dependency Versions:
           cffi: 1.5.2
       cherrypy: 3.6.0
       dateutil: 2.4.2
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.8
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: 0.21.1
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.6
   mysql-python: Not Installed
      pycparser: 2.10
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.13 (default, Jan 11 2017, 10:56:06) [GCC]
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 14.0.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.0.4
 
System Versions:
           dist: SuSE 12 x86_64
         locale: UTF-8
        machine: x86_64
        release: 4.4.73-5-default
         system: Linux
        version: SUSE Linux Enterprise Server  12 x86_64