Disappearing PRI from Key column after creating a trigger

I create a table on an empty database. Table is called Detail, has 4 columns with first one as a primary key:

CREATE TABLE `Detail` ( `idDetail` INT(11) NOT NULL AUTO_INCREMENT , `price` FLOAT NOT NULL DEFAULT '0' , `shipping` FLOAT NOT NULL DEFAULT '0' , `wholePrice` FLOAT NOT NULL DEFAULT '0' , PRIMARY KEY (`idDetail`)) ENGINE = InnoDB CHARACTER SET utf8;

I insert a single record and display column info:

INSERT INTO `Detail` (`idDetail`, `price`, `shipping`, `wholePrice`) VALUES (NULL, '5', '3', '8');

SHOW FULL COLUMNS FROM Detail;

There is PRI in KEY column of idDetail (as it should be). I then create a trigger on INSERT and display column info again:

CREATE TRIGGER `suma_I_Detail` BEFORE INSERT ON `Detail` FOR EACH ROW SET NEW.wholePrice = NEW.price + NEW.shipping;

SHOW FULL COLUMNS FROM Detail;

No change, PRI is still where it should be. I then execute an update on a single and only row. Update should neither activate the trigger, nor actually change anything (since the value in the row is the same as in the update).
After update I display column info once again:

UPDATE Detail SET price = 5 WHERE idDetail = 1;

SHOW FULL COLUMNS FROM Detail;

Suddenly PRI is missing from KEY column. I execute the very same update again and then show columns again.

UPDATE  Detail SET price = 5 WHERE idDetail = 1;

SHOW FULL COLUMNS FROM Detail;

PRI is back in the KEY column. If I then again execute update, PRI is gone, and then after another one is back. Same behaviour when I execute INSERT.

I then DROP the trigger, update the record and show columns again twice :

DROP TRIGGER IF EXISTS `suma_I_Detail`;

UPDATE Detail SET price = 5 WHERE idDetail = 1;

SHOW FULL COLUMNS FROM Detail;

UPDATE Detail SET price = 5 WHERE idDetail = 1;

SHOW FULL COLUMNS FROM Detail;

After the trigger is dropped, SHOW columns always correctly lists PRI in Key column for idDetail.