MySQL does not recognize possible_keys in join - mysql

Question

I tried to query by join 2 tables giao_vien and lop_mon.
The following is my tables structure:
CREATE TABLE `lop_mon` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`key_index` varchar(250) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`khoi_key_index` varchar(250) DEFAULT NULL,
`lop_key_index` varchar(250) DEFAULT NULL,
`gv_key_index` varchar(250) DEFAULT NULL,
`mon_key_index` varchar(250) DEFAULT NULL,
`ma_so` varchar(50) DEFAULT NULL,
`ma_phong` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`nam_hoc` int(11) NOT NULL,
`ma_truong` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`ma_khoi` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`ma_lop` varchar(50) NOT NULL,
`ma_giao_vien` varchar(50) NOT NULL,
`ma_mon_hoc` varchar(50) DEFAULT NULL,
`hoc_ky` tinyint(4) NOT NULL,
`guid` varchar(250) DEFAULT NULL,
`processid_monhoc` int(11) DEFAULT NULL,
`processid_giaovien` int(11) DEFAULT NULL,
`processid_canbo` int(11) DEFAULT NULL,
`source_data` varchar(50) DEFAULT NULL,
`ten_gv_chat_ph` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`ma_giao_vien_bgd` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`ma_lop_bgd` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`ma_mon_hoc_bgd` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
`gv_guid` char(36) DEFAULT NULL,
`truong_key_index` varchar(255) DEFAULT NULL,
`create_date` datetime DEFAULT NULL,
`updatedAt` datetime DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE,
UNIQUE KEY `key_UNIQUE` (`key_index`),
UNIQUE KEY `unique_lop_mon_gv_keyindex_hoc_ky` (`lop_key_index`,`gv_key_index`,`mon_key_index`,`hoc_ky`) USING BTREE,
KEY `lopmon_keyindex` (`mon_key_index`),
KEY `lopmon_idx_mamonhoc` (`ma_mon_hoc`),
KEY `idx_gv_key_index` (`gv_key_index`),
KEY `lopmon_idx_lop_key_index` (`lop_key_index`),
KEY `lopmon_idx_key_index` (`khoi_key_index`),
KEY `lopmon_idx_ma_truong` (`ma_truong`),
KEY `lopmon_idx_nam_hoc` (`nam_hoc`),
KEY `lopmon_idx_ma_khoi` (`ma_khoi`),
KEY `lopmon_idx_hoc_ky` (`hoc_ky`),
KEY `lopmon_idx_key_lop_mon` (`lop_key_index`,`mon_key_index`),
KEY `lopmon_idx_key_lop_gv` (`lop_key_index`,`gv_key_index`),
KEY `lopmon_idx_key_mon_gv` (`mon_key_index`,`gv_key_index`),
KEY `lopmon_idx_key_lop_mon_gv` (`lop_key_index`,`mon_key_index`,`gv_key_index`),
KEY `lopmon_idx_key_truong_lop_mon_gv` (`ma_truong`,`lop_key_index`,`mon_key_index`,`gv_key_index`),
KEY `lop_mon__index_gv_guid` (`gv_guid`),
KEY `lopmon_idx_truong_key_index` (`truong_key_index`)
) ENGINE=MyISAM AUTO_INCREMENT=23142297 DEFAULT CHARSET=latin1
My Query:
select * from giao_vien inner join lop_mon on lop_mon.gv_key_index = giao_vien.key_index
where giao_vien.key_index = 'shcm_2019_01645344018'
When I explain it:
MySQL does not recognize the key:
KEY `idx_gv_key_index` (`gv_key_index`)
I check another query:
SELECT * FROM enetviet.lop_mon where gv_key_index = 'shcm_2019_01645344018';
The key idx_gv_key_index is still work. I don't know what is the problem.