fileserver: s3fs - contents are ignored

Description of Issue

When using fileserver.s3fs, the contents of the bucket are effectively ignored, despite local cache synchronization successfully completing.

NOTE: This is running on python2, installed from https://repo.saltstack.com, to work around #53244.

Setup

/etc/salt/master.d/fileserver.conf:

file_roots:
  base:
    - /srv/salt

s3:
  keyid: KEY
  key: KEYID
  buckets:
    base:
      - test-salt
  service_url: nyc3.digitaloceanspaces.com
  https_enable: True

fileserver_backend:
  - roots
  - s3fs

/srv/salt/top.sls:

base:
  '*':
    - test

s3://test-salt/test.sls:

/etc/testfile:
  file.managed:
    - contents:
      - line1
      - line2

Steps to Reproduce Issue

1. Run salt '*' state.apply
2. See the following returned:

minion:
    Data failed to compile:
----------
    No matching sls found for 'test' in env 'base'

As a sidenote, /var/cache/salt/master/s3cache/buckets_files.cache appears to be mostly empty, despite the logs suggesting cache sync is successful:

buckets_files.cache:

(dp0
.

logs, repeatedly:

[INFO    ] Syncing local cache from S3...
[INFO    ] Sync local cache from S3 completed.

Versions Report

Salt Version:
           Salt: 2019.2.0

Dependency Versions:
           cffi: Not Installed
       cherrypy: unknown
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: 2.0.3
      gitpython: 2.1.8
          ioflo: Not Installed
         Jinja2: 2.10
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: 1.0.7
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.15rc1 (default, Nov 12 2018, 14:31:15)
   python-gnupg: 0.4.1
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: 2.0.3
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.2.5

System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-50-generic
         system: Linux
        version: Ubuntu 18.04 bionic

Reproduce the failure using /etc/salt/master.d/fileserver.conf, /srv/salt/top.sls, and the s3://test-salt/test.sls example, then inspect the fileserver.s3fs cache-sync entry point and /var/cache/salt/master/s3cache/buckets_files.cache. The work is done when the synchronized bucket contents are indexed and state.apply resolves the test SLS without the reported missing-SLS error.