FortiClient proactively defends against advanced attacks. Its tight integration with the Security Fabric enables policy-based automation to contain threats and control outbreaks. FortiClient is compatible with Fabric-Ready partners to further strengthen enterprises’ security posture.
This article describes when switching from off-net endpoint profile to on-net endpoint profile, VPN password is not saved in FortiClient. 

This can happen when off-net endpoint profile is configured with Remote Access feature while on the on-net endpoint profile, Remote Access feature is disabled

The workaround for this matter is by enabling Remote Access feature in both on-net and off-net endpoint profiles to keep VPN states (Auto Connect, Save Password, Always Up). 

On-Fabric endpoint profile:

Off-Fabric endpoint profile:

If Remote Access is disabled on On-net endpoint profiles, VPN tunnels and its states (Auto Connect, Save Password, Always Up) will be removed.