FortiOS 6.4.1 - Explicit proxy problem

[anru]

Hi all,
after upgrading from 6.2.3 to 6.4.1 fw version on my FG-500E, the explicit proxy no longer works well although there has been no configuration change.
With new firmare:

1. All users are no longer authenticated but can browse anyway
2. SNAT IP of proxy users is that of the listening interface and there is no possibility to change it with that of the outgoing interface of proxy policy.

With old firmare, instead,  all users are authenticated on ldap server and SNAT IP was that of outgoing interface of proxy policy.

Anyone has my same problem? There's a way to solve it?
Thanks a lot.