OK, combining that with
add_action('admin_head', 'admin_styles');
function admin_styles() {
?>
<style>
.acf-editor-wrap iframe {
min-height: 0;
}
</style>
<?php
}
?>
(in functions.php) solved it. (found it on this thread https://support.advancedcustomfields.com/forums/topic/how-to-set-a-custom-height-for-wysiwyg/)