The prompt specified by
FileServer.Settings.ShellPrompt is used for all new SSH shell sessions, but it does not affect existing shell sessions that were initialized with a different
ShellPrompt. This is not an issue, this is by design - once a session has been established, its environment can only be changed through the session itself.
What are you actually trying to achieve? Does it indeed require manipulating the shell prompt for all existing shell sessions, or would a capability to modify environment of individual shell sessions by sufficient to achieve your goals?