Thanks for the contribution!

Why will this fix your fatal error on the plugin? The change, as far as I can see, is just changing a variable name.

Hi, thanks for approving the pull request.
Here is my explanation.

The original code from SCF:

// Get raw value.
$value = acf_get_value( $post_id, $field );

// First format for rest API for backward compatibility.
$value = acf_format_value_for_rest( $value, $post_id, $field, $format );

// We keep this one for backward compatibility with existing code that expects the field value to be.
$fields[ $field['name'] ] = $value;
$fields[ $field['name'] . '_source' ] = array(
'label' => $field['label'],
'type' => $field['type'],
// This line re-formats the already formatted REST value instead of the raw value, which causes issues.
'formatted_value' => acf_format_value( $value, $post_id, $field ),
);

I fixed it by renaming the formatted REST value to $rest_value and leaving the original $value untouched. This way, when calling acf_format_value() to populate the formatted_value property of _source, it’s based on the raw value, not on the already formatted value:

// Format value for rest API for backward compatibility.
$rest_value = acf_format_value_for_rest( $value, $post_id, $field, $format );

// We keep this one for backward compatibility with existing code that expects the field value to be.
$fields[ $field['name'] ] = $rest_value;
$fields[ $field['name'] . '_source' ] = array(
'label' => $field['label'],
'type' => $field['type'],
'formatted_value' => acf_format_value( $value, $post_id, $field ),
);

In my plugin (Meta Field Block), I used the 'acf/rest/format_value_for_rest' filter to change the format of the REST value to support displaying all field types (even Group, Repeater, Flexible, Relationship, etc). When SCF re-format some multi-value field types like select, it treats the altered REST value as multiple values, which is why the error occurs.