Query posts for a multiselect

I want to query posts for a multiselect value and can’t get it to work. Here is my initial situation, which works in one place.

	label: Betrifft folgende Artists
	type: multiselect
	options: query
	query: site.page("artists").children
<?php foreach ($page->children() as $item): ?>
<?php $artist = $item->artists()->toPages(','); ?>
		<?php foreach($artist as $bla): ?>
		<a href="<?= $bla->url() ?>"><?= $bla->title() ?></a>
		<?php endforeach ?>
<?php endforeach ?>

But on another place …

I like to filter those news by a custom “artist” in the “artists” multiselect.
Intuitivley I tried this, suspecting that it would not work.

<?php $bla = $pages->find('news')->children()->filterBy('artists', 'in', 'Carl Andre') ?>

To check if it’s really just this secific filtering I tried it by title and it worked.

<?php $bla = $pages->find('news')->children()->filterBy('title', 'Rhabarber') ?>

I think I somehow need to split «artists» up but I have no idea how.

<?php $result = $pages->find('news')->children()->filter(function($child) {
  return $child->artists()->toPages(',')->findBy('title', 'Some title');

If you are actually on the single artist page and want to get all the news related to the artist, you can also do it like this:

<?php $result = $pages->find('news')->children()->filter(function($child) use($page) {
  return $child->artists()->toPages(',')->has($page);

Thank you so much!