CURRENT_TIMESTAMP produces zeros via trigger

+-------------------------+------------------+

| Variable_name           | Value            |

+-------------------------+------------------+

| innodb_version          | 5.6.32-79.0      |

| protocol_version        | 10               |

| slave_type_conversions  |                  |

| version                 | 10.1.19-MariaDB  |

| version_comment         | MariaDB Server   |

| version_compile_machine | x86_64           |

| version_compile_os      | Linux            |

| version_malloc_library  | bundled jemalloc |

| version_ssl_library     | YaSSL 2.4.2      |

| wsrep_patch_version     | wsrep_25.16      |

+-------------------------+------------------+

10 rows in set (0.00 sec)

CREATE TABLE tbl(

a TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,

b INT NOT NULL DEFAULT '123');

CREATE TABLE tbl2(

c TIMESTAMP NOT NULL);

DELIMITER $

CREATE TRIGGER tbl_before_insert BEFORE INSERT ON tbl FOR EACH ROW

BEGIN

SET @a = NEW.a;

SET @b = NEW.b;

INSERT INTO tbl2(c) VALUES(NEW.a);

END;$

DELIMITER ;

INSERT INTO tbl VALUES ();

mysql [localhost:10119] {msandbox} (test) > INSERT INTO tbl1 VALUES ();

Query OK, 1 row affected (0.01 sec)

mysql [localhost:10119] {msandbox} (test) > SELECT * FROM tbl1;

+---------------------+-----+

| a                   | b   |

+---------------------+-----+

| 2019-06-21 15:11:05 | 123 |

+---------------------+-----+

1 row in set (0.00 sec)

mysql [localhost:10119] {msandbox} (test) > SELECT * FROM tbl2;

+---------------------+

| c                   |

+---------------------+

| 0000-00-00 00:00:00 |

+---------------------+

1 row in set (0.00 sec)

mysql [localhost:10119] {msandbox} (test) > SELECT @a, @b;

+---------------------+------+

| @a                  | @b   |

+---------------------+------+

| 0000-00-00 00:00:00 |  123 |

+---------------------+------+

1 row in set (0.00 sec)