After upgrading the os from 5300-06 to 5300-11, this weird behaviour happened.
My system had another unix account, let's call it "abc", purposely configured its uid to 0 for working as a root with all its authorities. It has been run like this for years and worked completely fine. However after the upgrade, when abc's password expires, rather than prompting immediately after its successful login for a new password, it is now asking to change "root"'s password, and it really is changing root's password not abc's.
If I change abc's uid to another unique id, it will successfully says to change abc's password when prompted.
There are a lot of reasons why I cannot change abc's uid. So what I'm trying to find out is, why is this happening and how can I "fix" it? Is it really caused by the OS upgrade?