Ember CSS - Images not loading in production

I'm building an Ember site and I'm having problems with my images not loading in production.

I'm using the latest version. I've installed the SASS Module and have a very basic stylesheet in app\styles\app.scss

html {
    height: 100%;
    overflow-y: hidden;
}

body {
    background-image: url('/assets/images/StockSnap_8SAODL7HZ4.jpg');
    background-position: 50%;
    background-repeat: no-repeat;
    background-size: cover;
    height: 100%;
}

The SASS module is happily converting that into app.css

When I do a basic ember serve this works fine. However doing a ember deploy production --activate to AWS S3 things go awry.

All of my assets get fingerprinted as expected, so my image file becomes assets/Images/StockSnap_8SAODL7HZ4-4279bf0a502da08d183b81b67d479b40.jpg

However my app.css doesn't get updated and continues to look for assets/Images/StockSnap_8SAODL7HZ4.jpg

I guess I'm missing something obvious but not sure what!