When a member changes his password, his group memberships are reset.
Create a new group in ZMI called Test. Give Test the role Manager.
Go to plone and add yourself as a new user. Log out again.
In ZMI go to this new user and remove his Member role and make him a member of the group Test.
He will now have the inherited role of Manager.
Go to the portal again and log-in as this new user. Change the password and then log-out again.
Go to ZMI and go this new user. His membership for the group Test is gone! He will have the explicit role of Manager. I also have seen that he has no roles at all or simply the Member role.
In any way, this is a very serious issue.