2019.2 salt-ssh saltutil.sync_all/grains removes custom grains that were working fine

Description of Issue

Custom grains synced first time, lost after saltutil.sync_all or sync_grains. Using salt-ssh but not certain if a specific issue to salt-ssh or not.

Setup

I'm using salt-ssh. I have some grains in _grains/. That work normally and show up in grains.items.

Steps to Reproduce Issue

If I do a saltutil.sync_all, I can see the grains vanish in the /var/tmp/.root_whatever_salt directory. If I do grains.items, they don't report back. The only fix I've found is to blow away the /var/tmp/.root_whatever_salt directory and do another salt-ssh.

Versions Report

 #$ salt-call --versions-report
Salt Version:
           Salt: 2019.2.0
 
Dependency Versions:
           cffi: 1.12.3
       cherrypy: Not Installed
       dateutil: 2.5.3
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10.1
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.8
   mysql-python: Not Installed
      pycparser: 2.19
       pycrypto: Not Installed
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.5.3 (default, Sep 27 2018, 17:25:39)
   python-gnupg: Not Installed
         PyYAML: 5.1.1
          PyZMQ: Not Installed
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.4.3
            ZMQ: Not Installed
 
System Versions:
           dist: debian 9.9 
         locale: UTF-8
        machine: x86_64
        release: 4.9.0-9-amd64
         system: Linux
        version: debian 9.9 

I have tested this with a Debian 9 master and both Debian 9 and 10 minions.

Related: #39671

Thank you!

Start by reproducing the issue with salt-ssh, custom grains in _grains/, saltutil.sync_all or sync_grains, and grains.items; inspect the cached /var/tmp/.root_whatever_salt directory when the grains disappear. Done means custom grains remain available after synchronization without manually deleting that cached directory.