[BUG] Minion returns randomly missing in batch mode

Description

Sometimes some minion returns are missing in the output of a batch mode command.
For a command like salt -C 'I@salt:minion' --out=json -b'70%' cmd.run "salt-call cmd.run 'echo OK'" the output might look like:

....<<skip>>....
{
    "jid": "20200919150026255687",
    "retcode": 0,
    "node-11": "[INFO    ] Executing command 'echo OK' in directory '/root'\nlocal:\n    OK"
}
{
    "jid": "20200919150027708040",
    "node-12": "[INFO    ] Executing command 'echo OK' in directory '/root'\nlocal:\n    OK",
    "retcode": 0
}
{
    "node-6": {}
}

Although all minions are alive, in some cases some outputs are missing. The minions are not overloaded and there are no networking issues. Oftentimes, in subsequent runs the same minion outputs are missing. When the issue occurs, a "find_job" job with the target of the missing minion appears in jobs.list_jobs:

20200919162151938541:
    ----------
    Arguments:
        - 20200919162051760278
    Function:
        saltutil.find_job
    StartTime:
        2020, Sep 19 16:21:51.938541
    Target:
        - node-6
    Target-type:
        list
    User:
        salt-user

Setup
This particular setup has 23 minions but the configuration is tuned for much bigger environments.
master.conf:

sock_pool_size: 15
zmq_backlog: 3000
gather_job_timeout: 30
worker_threads: 20
timeout: 60
max_open_files: 15000
state_output: changes
pillar_opts: False
pillar_safe_render_error: True
auto_accept: True
max_event_size: 100000000

minion.conf:

master: 10.0.0.1
id: node-6
acceptance_wait_time: 10
acceptance_wait_time_max: 60
auth_timeout: 180
master_tries: -1
max_event_size: 100000000
random_reauth_delay: 270
recon_default: 1000
recon_max: 60000
recon_randomize: True

Steps to Reproduce the behavior
It's enough to run salt -C 'I@salt:minion' --out=json -b'70%' cmd.run "salt-call cmd.run 'echo OK'" several times.
On my setup with 23 minions it gives almost 50% reproduction rate.

Expected behavior
All returns from all minions are present in the output.

Versions Report

Initially caught on 2017.7.x series, reproduced also on 3000.3.

Salt Version:
           Salt: 3000.3

Dependency Versions:
           cffi: 1.9.1
       cherrypy: 3.5.0
       dateutil: 2.4.2
      docker-py: 1.9.0
          gitdb: Not Installed
      gitpython: Not Installed
         Jinja2: 2.8
        libgit2: Not Installed
       M2Crypto: 0.21.1
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.12 (default, Nov 12 2018, 14:36:49)
   python-gnupg: 0.3.8
         PyYAML: 3.11
          PyZMQ: 15.2.0
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.1.4

System Versions:
           dist: Ubuntu 16.04 xenial
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-43-generic
         system: Linux
        version: Ubuntu 16.04 xenial

Additional context

Only the batch client is affected, does not reproduce with any other client.