Actually it has really nothing to do with the OS version as much as it does Cpanel itself ...
If I set the the password from WHM or Cpanel on any account or any server
then only the first 8 characters get encoded into the password.
If I open up an SSH window and use the 'passwd' command to change the
user's password from the shell then all characters are kept in the password
This tells me that Cpanel programmed in an 8 character length in their field
and are truncating any additional characters passed for passwords.