Problem with MODIFY COLUMN

[MySQL 5.7]

I found that the queries

CREATE TABLE `test` (
  `test_id` int(16) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

ALTER TABLE `test`
  ADD PRIMARY KEY (`test_id`);

ALTER TABLE `test`
  MODIFY `test_id` int(16) AUTO_INCREMENT;

seem to be parsed in a strange way. After importing the above as a script and viewing the generated table as SQL it shows

CREATE TABLE test (
  test_id int(16) NULL AUTO_INCREMENT,
  test_id int(16) NULL AUTO_INCREMENT,
  CONSTRAINT test_pk PRIMARY KEY (test_id)
) ENGINE InnoDB CHARACTER SET utf8mb4 COLLATE utf8mb4_bin;

Which is not what I had in mind but I cannot figure out the problem. 

MySQL Workbench parses this in the wrong way, too - ignoring the MODIFY completely.

PHPMyAdmin on the other hand does it correctly.

Any thoughts on this? :-)

Thanks!