Sometimes, an array is passed to the get_callback provided to register_rest_field() instead of an object

When using register_rest_field() to add fields to terms in the WordPress REST API, the $object sent to your get_callback function will be an array instead of an object like the documentation describes.

An object will still be passed, sometimes, too. Here is an example of how I work around this problem:

	function add_api_term_fields() {

		//location-phone-hours
		register_rest_field( 'location', 'location-phone-hours', array(
			'get_callback'    => array( $this, 'get_term_meta_via_rest' ),
			'update_callback' => array( $this, 'set_term_meta_via_rest' ),
			'schema'          => array(
				'description' => __( 'An array of phone numbers and hours of operation for this location.', 'inventory-portal' ),
				'type'        => 'string',
				'context'     => array( 'view', 'edit' ),
			),
		) );
	}

	static function get_term_meta_via_rest( $term, $attr, $request, $object_type ) {
		$term_id = 0;

		if(  is_array( $term ) ) {
			$term_id = $term['id']
		} else {
			$term_id = $term->term_id;
		}

		return maybe_serialize( get_term_meta( $term_id, $attr, true ) );
	}

Why is this happening? See the definition of prepare_item_for_response() method on line 683 of wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php. The call to add_additional_fields_to_object() on line 725 is the method that passes an array to your get_callback.

Perhaps this is an acceptable design. I believe the only reference to the data type of $object in the documentation is this comment. It certainly feels like a bug.