Changing the password of mail account does not change the password of its alias.
The behavior is caused by software issue #PPA-2920.
The fix is planned to be included in one of the future product updates.
To workaround the issue, recreate the mail alias after the password of original mail account is changed.
Another workaround: Login to Customer Panel > Websites > Select domain alias name > Disable and Enable mail service. On this action PA synchronizes passwords for primary domain and it's aliases.