runner switches between cmd.exe and bash ranadomly

When a CI build is triggered by a git check-in, the multi-runner, sometimes it runs the the stages in cmd.exe, and other times uses bash.

Gitlab version: hosted (gitlab.com) Build slave: Win 2012

C:\Multi-Runner>gitlab-ci-multi-runner-windows-amd64.exe -v
Version:      1.5.3
Git revision: fb49c47
Git branch:   1-5-stable
GO version:   go1.6.3
Built:        Tue, 13 Sep 2016 19:39:13 +0000
OS/Arch:      windows/amd64

CMD.EXE Output:

Running with gitlab-ci-multi-runner 1.5.3 (fb49c47)
Using Shell executor...
Running on WIN-O1T3QNC4INP...
Fetching changes...
Removing packages/
HEAD is now at 52bc22a ci
From https://gitlab.com/bke/main
   52bc22a..de9ae38  master     -> origin/master
Checking out de9ae384 as master...
$ set GIT_CURL_VERBOSE=1
$ set GIT_TRACE=1
$ whoami
nt authority\system
$ set
ALLUSERSPROFILE=C:\ProgramData

bash Output:

Running with gitlab-ci-multi-runner 1.5.3 (fb49c47)
Using Docker executor with image ruby:2.1 ...
Pulling docker image ruby:2.1 ...
Running on runner-8a2f473d-project-1609461-concurrent-0 via runner-8a2f473d-machine-1474586336-61eac341-digital-ocean-4gb...
Cloning repository...
Cloning into '/builds/bke/main'...
Checking out 9a97a0fb as master...
$ set GIT_CURL_VERBOSE=1
$ set GIT_TRACE=1
$ whoami
root
$ set
BASH=/bin/bash

.gitlab-ci.yml:

variables:
  PROJECT_NAME: "MyCompany"
before_script:
  - whoami
  - set
stages:
  - build
  - test
build:
  stage: build
  script:
  - echo "Release build..."
  artifacts:
    untracked: true
test:
  stage: test
  script:
  - echo "Starting tests"
  dependencies:
  - build

config.toml:

concurrent = 1
check_interval = 0
[[runners]]
  name = "build-1"
  url = "https://gitlab.com/ci"
  token = "bla bla bla"
  executor = "shell"
  shell = "cmd"