MySQL user privileges lost on reboot
This is a strange behaviour!
Once user is created you can make sure of that by reviewing the grants for it.

show grants for 'username'@'hostname';

Privileges are persistent, you don't need any additional step to make them stay.

Check if my.cnf has something like --init-file or --skip-grant-tables. Also, how mysql is started? Does your root password get reset as well? Are you sure you're connected to the right mysql?