[BUG] system.reboot_required_witnessed sticks on True after setting it and rebooting

Description
system.get_reboot_required_witnessed returns always true once set.

Setup

• QEMU/kvm virtual machine, master and minion on same machine
• happens also with master and minion on different machines

Config files:

/etc/salt/master:
interface: 127.0.0.1

/etc/salt/minion:
master: 127.0.0.1
id: localminion

Steps to Reproduce the behavior

• start and enable both master and minion services
• accept minion key

[tqre@salt-debug ~]$ sudo salt '*' system.get_reboot_required_witnessed
localminion:
    False
[tqre@salt-debug ~]$ sudo salt '*' system.set_reboot_required_witnessed
localminion:
    True
[tqre@salt-debug ~]$ sudo reboot
Connection to 192.168.0.10 closed by remote host.
Connection to 192.168.0.10 closed.
...reconnect after machine has booted...
[tqre@salt-debug ~]$ sudo salt '*' system.get_reboot_required_witnessed
localminion:
    True

Expected behavior
system.reboot_required_witnessed should reset to False after reboot.

Versions Report

Salt Version:
           Salt: 3001

Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
         Jinja2: 2.11.2
        libgit2: Not Installed
       M2Crypto: 0.36.0
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 1.0.0
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: Not Installed
   pycryptodome: 3.9.7
         pygit2: Not Installed
         Python: 3.8.5 (default, Jul 27 2020, 08:42:51)
   python-gnupg: Not Installed
         PyYAML: 5.3.1
          PyZMQ: 19.0.1
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.3.2

System Versions:
           dist: arch
         locale: utf-8
        machine: x86_64
        release: 5.7.12-arch1-1
         system: Linux
        version: Arch Linux

Start by locating the implementations of system.get_reboot_required_witnessed and system.set_reboot_required_witnessed, then trace how their state survives the reboot described in the reproduction. Re-run the commands before and after reboot; done means the getter returns False after the minion restarts while still returning True immediately after setting the witness.