Relative includes are not working when being called with state.sls <directory>/<slsfile>

Description of Issue

When calling a state with for example salt mx3 state.sls mailserver/postfix relative includes are not working. When called as salt mx3 state.sls mailserver.postfix they do work.

Setup

mailserver/postfix.sls:

include:
  - .vmail_users


# Make sure the postfix and mail-utils package are installed.
postfix_package:
  pkg.installed:
    - names:
      - postfix
      - mailutils

mailserver/vmail_users.sls:

vmail_group:
  group.present:
    - name: vmail
    - gid: 404

Steps to Reproduce Issue

salt mx3 state.sls mailserver/postfix results in:

# salt mx3 state.sls mailserver/postfix
mx3:
    Data failed to compile:
----------
    Specified SLS vmail_users in saltenv base is not available on the salt master or through a configured fileserver

salt mx3 state.sls mailserver.postfix works as expected

Versions Report

Salt Version:
Salt: 2019.2.2

Dependency Versions:
cffi: 1.11.5
cherrypy: Not Installed
dateutil: 2.4.2
docker-py: Not Installed
gitdb: 0.6.4
gitpython: 1.0.1
ioflo: Not Installed
Jinja2: 2.8
libgit2: Not Installed
libnacl: Not Installed
M2Crypto: Not Installed
Mako: 1.0.3
msgpack-pure: Not Installed
msgpack-python: 0.4.6
mysql-python: Not Installed
pycparser: 2.18
pycrypto: 2.6.1
pycryptodome: Not Installed
pygit2: Not Installed
Python: 2.7.12 (default, Oct 8 2019, 14:14:10)
python-gnupg: 0.3.8
PyYAML: 3.11
PyZMQ: 15.2.0
RAET: Not Installed
smmap: 0.9.0
timelib: Not Installed
Tornado: 4.2.1
ZMQ: 4.1.4

System Versions:
dist: Ubuntu 16.04 xenial
locale: UTF-8
machine: x86_64
release: 4.4.110-mainline-rev1
system: Linux
version: Ubuntu 16.04 xenial