Why "Disconnected Session Timer" policy doesn't work on Windows Server OS when applied from Citrix Studio's Policy tab but works on Windows Desktop OS?
Why does the same policy work when applied from MS GPO on the Same Server OS?

Instructions

As we know the major difference between a Windows Server OS and Windows Desktop OS is that the server OS can take multiple sessions however Desktop OS can only take one session at a time because of the Listeners. 

When we connect to a Desktop OS using Citrix, Port ICA is responsible for the connection and Session timeout policies from CItrix Studio will work as expected.
When we connect to a Server OS using Citrix, precedence is given to RDP as the server is acting as a session host and ICA protocol is running on top of RDP protocol and that is the reason we recommend to apply the Session Disconnect Timeout policy from MS GPO for Server OS. However, Session Idle timer policy will still apply from Citrix Studio for Server OS.