Why are my database usernames prefixed by the full username?
By default, the databases created through the cPanel interface are expected to limit themselves to 8 character prefixes. This helps administrators easily identify which databases are tied to their respective cPanel accounts.
For MariaDB 10.x and MySQL 5.7.9, the maximum username length changed. For MariaDB, the maximum username length is 80 characters. For MySQL, 5.7.9 the maximum username length is 30 characters. This restriction does still apply in MySQL 8.