How do I see the value of one field of a multi-item field in views formatter
You don't need to search for another method, Field::getValue() returns the array you are looking for:

if ($view-&gt;field[$field_name]-&gt;multiple) {
  $field_array = $view-&gt;field[$field_name]-&gt;getValue($row);
}


I don't know if there is a documentation, but you can see this in code:

https://api.drupal.org/api/drupal/core!modules!views!src!Plugin!views!field!Field.php/function/Field%3A%3AgetValue/8.2.x

Difference between field values from view result and entity

If you have a standard multivalue field like text or number in a view and don't change any options, getValue($row) returns a one-dimensional array with the values from the main property of the field. If not you have somehow configured the view so that it shows a different result.

You can although get the original entity field data, which is independent from the configuration of the view. For this the field needs to be in the row entity:

$field_array = $row-&gt;_entity-&gt;{$field_name}-&gt;getValue();


This is then a two-dimensional array with multiple values and multiple properties.