ACF has two fields that must be present. These are the field value and field key reference.
So for example let’s say the the field name is “gallery” and the field key is “field_123456”
A gallery field is an array of the attachment ID values from the media folder
'meta_input = array(
'gallery' => array($attachment_id), // field value
'_gallery' => 'field_123456' // field key reverence
)
