Solved it…
In my functions.php I should have put:
array(
'title' => 'Hidden Element',
'inline' => 'span',
'classes' => 'hiddenElement',
'wrapper' => false,
),
not
array(
'title' => 'Hidden Element',
'block' => 'span',
'classes' => 'hiddenElement',
'wrapper' => false,
),
as here: http://www.wpbeginner.com/wp-tutorials/how-to-add-custom-styles-to-wordpress-visual-editor/