Sounds like you have some proxy based UTM profiles enabled which makes
FortiGate running in proxy mode to respond TCP connection on behalf of
the server. If you only have flow based UTM profiles in the affected
firewall policy, or don't enable UTM, t...
Please either send a request to ips@fortinet.com, or file mantis bug in
"IPS Engine & Sigs" to request Psiphon block without SSL deep
inspection. IPS analyst team should be able to look into the
feasibility.
Flow mode (done in IPS engine) doesn't rely on port numbers for security
inspection. As long as the relevant protocol is enabled, all traffic
will be inspected for protocol recognization first. As for HTTPS, it
depends on SSL deep inspection settings...