Initialize root password on MySQL 5.7 - mysql

Question

I'm installing MySQL 5.7 on a new Ubuntu 16.04 server. Since my installation uses Ansible, no root password is defined at installation time, and I set it up later with the mysql_user role in ansible, but the root password stays empty.
I therefore tried to run the command directly on MySQL, without Ansible, but I get the same behaviour : no error on the alter user command but password is not set.
My server configuration :
OS :
Distributor ID: Ubuntu
Description: Ubuntu 16.04.1 LTS
Release: 16.04
Codename: xenial
Database : MySQL 5.7.12-0ubuntu1.1
Step to reproduce :
Install mysql-server-5.7 package without providing a root password.
Connect to mysql from the system root account
And then run the mysql commands :
mysql> select authentication_string from user where host='localhost' and user='root';
+-----------------------+
| authentication_string |
+-----------------------+
| |
+-----------------------+
1 row in set (0,00 sec)
mysql> alter user root#localhost identified by 'mypassword';
Query OK, 0 rows affected (0,00 sec)
mysql> select authentication_string from user where host='localhost' and user='root';
+-----------------------+
| authentication_string |
+-----------------------+
| |
+-----------------------+
1 row in set (0,00 sec)
However, running the same command with a hashed value works :
mysql> ALTER USER 'root'#'localhost' IDENTIFIED WITH 'mysql_native_password' AS '*D7B8FEAED940F8F2A0055864C0A59782B4BCED02';
mysql> select authentication_string from user where host='localhost' and user='root';
+-------------------------------------------+
| authentication_string |
+-------------------------------------------+
| *D7B8FEAED940F8F2A0055864C0A59782B4BCED02 |
+-------------------------------------------+
1 row in set (0,00 sec)
And then, I can change the password with the original request :
mysql> alter user root#localhost identified by 'mypassword';
Query OK, 0 rows affected (0,00 sec)
mysql> select authentication_string from user where host='localhost' and user='root';
+-------------------------------------------+
| authentication_string |
+-------------------------------------------+
| *FABE5482D5AADF36D028AC443D117BE1180B9725 |
+-------------------------------------------+
1 row in set (0,00 sec)
Why does the "unhashed" version not work the first time ? Am I missing something or is this a bug in this version of MySQL ?