Mysql – Query ignores index resulting full table scan

I have two tables from django in mysql 5.7, auth_user and account_account:

CREATE TABLE `auth_user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(150) NOT NULL,
  `first_name` varchar(30) NOT NULL,
  `last_name` varchar(30) NOT NULL,
  `email` varchar(75) NOT NULL,
  `password` varchar(128) NOT NULL,
  `is_staff` tinyint(1) NOT NULL,
  `is_active` tinyint(1) NOT NULL,
  `is_superuser` tinyint(1) NOT NULL,
  `last_login` datetime DEFAULT NULL,
  `date_joined` datetime NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`),
  KEY `is_staff` (`is_staff`)
) ENGINE=InnoDB AUTO_INCREMENT=4438 DEFAULT CHARSET=utf8;

CREATE TABLE `account_account` (
  `user_ptr_id` int(11) NOT NULL,
  `org_form` smallint(5) unsigned DEFAULT NULL,
  `org_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  `contact_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  `balance` decimal(15,2) NOT NULL,
  `total_debt` decimal(15,2) NOT NULL,
  `contact_email` varchar(255) NOT NULL,
  `phone` varchar(255) NOT NULL,
  `mob_phone` varchar(255) NOT NULL,
  `other_phone` varchar(255) NOT NULL,
  `delivery_methods` smallint(5) unsigned NOT NULL,
  `delivery_address` longtext NOT NULL,
  `payment_method` smallint(5) unsigned NOT NULL,
  `person_type` smallint(5) unsigned NOT NULL,
  `client_type` smallint(5) unsigned NOT NULL,
  `city` smallint(5) unsigned NOT NULL DEFAULT '1',
  `patronymic` varchar(255) NOT NULL,
  `can_delay` tinyint(1) NOT NULL,
  `status` smallint(5) unsigned NOT NULL,
  `block_reason` varchar(255) NOT NULL,
  `created` datetime NOT NULL,
  `modified` datetime NOT NULL,
  `manager_id` int(11) DEFAULT NULL,
  `wholesale_level_id` int(11) DEFAULT NULL,
  `markup_type` smallint(5) unsigned NOT NULL,
  `org_markup_level_id` int(11) DEFAULT NULL,
  `use_vat` tinyint(1) NOT NULL,
  `last_order_date` datetime NOT NULL,
  `group_id` int(11) DEFAULT NULL,
  `is_partner` tinyint(1) NOT NULL,
  `partner_markup` decimal(15,2) DEFAULT NULL,
  `call_markup` decimal(15,2) DEFAULT NULL,
  `delivery_price` decimal(15,2) DEFAULT NULL,
  `city_id` int(11) NOT NULL,
  `unloading_at` time NOT NULL,
  `site` varchar(200),
  `ip_address_id` varchar(20) DEFAULT NULL,
  `ip_restriction` tinyint(1) NOT NULL,
  `visualisation_id` int(11),
  `_express_delivery_price` int(10) unsigned DEFAULT NULL,
  `is_express_delivery` tinyint(1) NOT NULL,
  `payment_type` smallint(5) unsigned NOT NULL,
  PRIMARY KEY (`user_ptr_id`),
  KEY `account_account_501a2222` (`manager_id`),
  KEY `account_account_7c811b3c` (`wholesale_level_id`),
  KEY `group_id_refs_id_35dd1116` (`group_id`),
  KEY `city_id_refs_id_96652cf` (`city_id`),
  KEY `account_account_ip_address_id_e42ae8c3_uniq` (`ip_address_id`),
  KEY `account_ac_visualisation_id_cd5729f6_fk_account_visualisation_id` (`visualisation_id`),
  CONSTRAINT `account_ac_visualisation_id_cd5729f6_fk_account_visualisation_id` FOREIGN KEY (`visualisation_id`) REFERENCES `account_visualisation` (`id`),
  CONSTRAINT `account_accou_ip_address_id_e42ae8c3_fk_account_ipadress_address` FOREIGN KEY (`ip_address_id`) REFERENCES `account_ipadress` (`address`),
  CONSTRAINT `account_account_city_id_2ab1a62f_fk_classifiers_city_id` FOREIGN KEY (`city_id`) REFERENCES `classifiers_city` (`id`),
  CONSTRAINT `group_id_refs_id_35dd1116` FOREIGN KEY (`group_id`) REFERENCES `account_clientgroup` (`id`),
  CONSTRAINT `user_ptr_id_refs_id_17b5ed9e` FOREIGN KEY (`user_ptr_id`) REFERENCES `auth_user` (`id`),
  CONSTRAINT `wholesale_level_id_refs_id_369bcb86` FOREIGN KEY (`wholesale_level_id`) REFERENCES `classifiers_wholesalelevel` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

I'm trying to do query:

explain     SELECT  `auth_user`.`id`, `account_account`.`contact_name`, `account_account`.`contact_email`,
            `account_account`.`phone`
        FROM  `account_account`
        INNER JOIN  `auth_user`  ON (`account_account`.`user_ptr_id` = `auth_user`.`id`)
        WHERE  (`auth_user`.`is_staff` = 0)
        LIMIT  16;

+----+-------------+-----------------+------------+--------+------------------+---------+---------+-----------------------------------+------+----------+-------------+
| id | select_type | table           | partitions | type   | possible_keys    | key     | key_len | ref                               | rows | filtered | Extra       |
+----+-------------+-----------------+------------+--------+------------------+---------+---------+-----------------------------------+------+----------+-------------+
|  1 | SIMPLE      | account_account | NULL       | ALL    | PRIMARY          | NULL    | NULL    | NULL                              | 3741 |   100.00 | NULL        |
|  1 | SIMPLE      | auth_user       | NULL       | eq_ref | PRIMARY,is_staff | PRIMARY | 4       | raros.account_account.user_ptr_id |    1 |   100.00 | Using where |
+----+-------------+-----------------+------------+--------+------------------+---------+---------+-----------------------------------+------+----------+-------------+

It shows that account_account does full scan even if we do not do any filtering on account, only join. Can you explain to me two things:

1. why mysql does not take is_staff index for initial filtering? we have two indexes on auth_user: primary and is_staff
2. why mysql does full scan on account_account, even if we do not do any filtering for account_account table?

Thanks in advance!