Comment 7 for bug 67521

I confirm the fact that KWallet loses new password after session restart
(actually I believe it does not save new passwords upon session closing)
and I had already filed a bug for it at the KDE bugtracking system:

http://bugs.kde.org/show_bug.cgi?id=136411

It also cites a different workaround to save passwords:
run "dcop kded kwalletd closeAllWallets" in a terminal before quitting.