Multiple select not working in drupal8
I had same issue , but i solved like this

$form['fieldset']['select-user'] = array(
   '#type' =&gt; 'select',
   '#title' =&gt; t('Select a user'),
   '#options' =&gt; select_user_dropdown_options(),
   '#description' =&gt; t('Select a user'),
   '#size' =&gt; 10,
   '#multiple' =&gt; TRUE,
   '#attributes' =&gt; array(
      'id' =&gt; 'edit-select-user',
      'style' =&gt; 'width:600px',
   )
);


Code:


Form:


Output in submit: