How to set value for multivalue field in drupal 8 programmatically
I would rewrite your code like this.
N.B.: This an edited version of the original answer, following some helpful points coming from the comments.

$poll = Poll::load($pollid);

$poll-&gt;question-&gt;setValue(['value' =&gt; 'test']);

$target_ids = array(13,14,15);
foreach($target_ids as $target_id){
  $poll-&gt;choice-&gt;appendItem($target_id);
}
$poll-&gt;save();


Hope this does it!