wp_enqueue_style places the output CSS directly in the HTML

I am using wp_register_style and wp_enqueue_style to add stylesheets to my custom theme. Locally it works as expected, however, on my staging machine instead of adding a link to the CSS file I have specified, it places the CSS directly in my HTML, like:

<style id="titilium-font-css" media="all">
  /* CSS styles here */
</style>

Here is how I enqueue my styles:

add_action( 'wp_enqueue_scripts', array( $this, 'register_styles' ) );

function register_styles() {
  wp_register_style( 'titilium-font', 'http://fonts.googleapis.com/css?family=Titillium+Web:400,600,700' );
  wp_enqueue_style( 'titilium-font' );
}

I expect the output to be (and it is locally):

<link rel="stylesheet" id="titilium-font-css" href="http://fonts.googleapis.com/css?family=Titillium+Web%3A400%2C600%2C700&amp;ver=4.0.2" type="text/css" media="all">

Any hints what to look for on the staging machine in order to solve this?