gulp-preprocess and environment variables on Windows 10

I am building a js app on OSX, and Windows 10, both running Node.js v4.2.x.

As part of my build process I run

BUILD_TYPE={my-build-type} gulp build

Which builds a particular configuration of my app, using gulp-preprocess.

The relevant part of my gulp file looks like

var env = process.env;

...
.pipe(plug.preprocess({context: { BUILD_TYPE: env.BUILD_TYPE}}))
...

The relevant part of my HTML looks like

<!-- @if BUILD_TYPE='live' -->
    <script src="config/live.js"></script>
<!-- @endif -->

<!-- @if BUILD_TYPE='dev' -->
    <script src="config/dev.js"></script>
<!-- @endif -->

<!-- @ifndef BUILD_TYPE -->
    <script src="config/dev.js"></script>
<!-- @endif -->

<p>build type - <!-- @echo BUILD_TYPE --></p>

On OSX this works great, running BUILD_TYPE=dev gulp build results in:

<script src="config/dev.js"></script>

<p>build type - dev</p>

However, on Windows 10, using Node.js command prompt,

set BUILD_TYPE=dev && gulp build 

results in:

<p>build type - dev</p>

With no script tags appearing! It's clearly recognised the environment variable, as it's printed by @echo BUILD_TYPE. However, none of the script elements are present.

Any clues as to why this might be happening would be much appreciated!

Oddly, if I change my gulpfile to:

...
.pipe(plug.preprocess({context: { BUILD_TYPE: 'dev'}}))
...

I get the expected output:

<script src="config/dev.js"></script>

<p>build type - dev</p>

Further still, if I log the variable before invoking preprocess:

OSX

console.log(env.BUILD_TYPE);
//dev
console.log(typeof env.BUILD_TYPE);
//string
console.log(env.BUILD_TYPE === 'dev');
//true

Windows 10

console.log(env.BUILD_TYPE);
//dev
console.log(typeof env.BUILD_TYPE);
//string
console.log(env.BUILD_TYPE === 'dev');
//false!