server occasionally does not not use correct 'DEFAULT CURRENT_TIMESTAMP' on insert

CREATE TABLE `xyz` (

  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,

...

  `start_date` datetime DEFAULT NULL,

...

  `last_modified` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

...

  `created_on` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,

...

) ENGINE=InnoDB AUTO_INCREMENT=5589824 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

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

| ordinal_position | column_name                      | column_default    | extra                       |

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

|                1 | id                               | NULL              | auto_increment              |

...

|                9 | start_date                       | NULL              |                             |

...

|               21 | last_modified                    | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |

...

|               32 | created_on                       | CURRENT_TIMESTAMP |                             |

#Q> /*trace_id:b187ded0caa2296c,span_id:b26af9f59f7c5974,application:abc:abc,action:doPost,timestamp:1521559772*/ INSERT  INTO xyz.xyz (code,name,start_date,timezone,currency,xyz1,xyz2,xyz3,xyz4,xyz5,xyz6,xyz7,deleted) VALUES ('----','----','2018-03-20 15:29:30','EST5EDT','USD','----','0','0','2524','2167680','1','2','0')

#180320 15:29:32 server id 371  end_log_pos 317969730   Table_map: `xyz`.`xyz` mapped to number 1272 (has triggers)

# at 317969730

#180320 15:29:32 server id 371  end_log_pos 317969947   Write_rows: table id 1272 flags: STMT_END_F

### INSERT INTO `xyz`.`xyz`

### SET

###   @1=5555902

###   @2=1

###   @3=0

###   @4='----'

###   @5='----'

###   @6=101668685

###   @7=2167680

###   @8=2524

###   @9=2018-03-20 15:29:30

###   @10=NULL

###   @11='EST5EDT'

###   @12=NULL

###   @13=0

###   @14=0

###   @15=0

###   @16=1

###   @17=1

###   @18=0.000000

###   @19=2

###   @20=2

###   @21=0

###   @22=1

###   @23=NULL

###   @24=NULL

###   @25='USD'

###   @26=1

###   @27=0

###   @28=0.000

###   @29=NULL

###   @30=NULL

###   @31=NULL

###   @32=1521559772

###   @33=NULL

###   @34=NULL

###   @35=0

###   @36=0

###   @37=0

###   @38=1

###   @39=2

###   @40=NULL

###   @41=1

###   @42=1

###   @43=NULL

###   @44=0

###   @45=NULL

###   @46=0

# at 317969947