MySQL deadlock routine: inconsistent batch insertion order under unique index

CREATE TABLE `t1` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `a` varchar(5),
 `b` varchar(5),
 PRIMARY KEY (`id`),
 UNIQUE KEY `uk_name` (`a`,`b`)
);

mysql> select * from information_schema.innodb_locks;
+-------------+-------------+-----------+-----------+------------+------------+------------+-----------+----------+-----------+
| lock_id | lock_trx_id | lock_mode | lock_type | lock_table | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------+-------------+-----------+-----------+------------+------------+------------+-----------+----------+-----------+
| 31AE:54:4:2 | 31AE | S | RECORD | `d1`.`t1` | `uk_name` | 54 | 4 | 2 | '1', '1' |
| 31AD:54:4:2 | 31AD | X | RECORD | `d1`.`t1` | `uk_name` | 54 | 4 | 2 | '1', '1' |
+-------------+-------------+-----------+-----------+------------+------------+------------+-----------+----------+-----------+

------------------------
LATEST DETECTED DEADLOCK
------------------------
181101 9:48:36
*** (1) TRANSACTION:
TRANSACTION 3309, ACTIVE 215 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 376, 2 row lock(s), undo log entries 2
MySQL thread id 2, OS thread handle 0x70000a845000, query id 58 localhost root update
insert into t1(a, b)values("2", "2")
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 55 page no 4 n bits 72 index `uk_name` of table `d1`.`t1` trx id 3309 lock mode S waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 1; hex 32; asc 2;;
 1: len 1; hex 32; asc 2;;
 2: len 4; hex 80000002; asc ;;

*** (2) TRANSACTION:
TRANSACTION 330A, ACTIVE 163 sec inserting
mysql tables in use 1, locked 1
3 lock struct(s), heap size 376, 2 row lock(s), undo log entries 2
MySQL thread id 3, OS thread handle 0x70000a888000, query id 59 localhost root update
insert into t1(a, b)values("1", "1")
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 55 page no 4 n bits 72 index `uk_name` of table `d1`.`t1` trx id 330A lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 1; hex 32; asc 2;;
 1: len 1; hex 32; asc 2;;
 2: len 4; hex 80000002; asc ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 55 page no 4 n bits 72 index `uk_name` of table `d1`.`t1` trx id 330A lock mode S waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 1; hex 31; asc 1;;
 1: len 1; hex 31; asc 1;;
 2: len 4; hex 80000001; asc ;;

*** WE ROLL BACK TRANSACTION (2)