you're asking about the behavior of the "fiddler.network.egress.ip" preference. Is that correct? Can you more completely describe your scenario?
This preference affects the local binding of the endpoint when a new TCP/IP connection is established. Taking a total shot in the dark, would I be correct in guessing that you're trying to change the value of this preference between requests
without restarting Fiddler?
If so, the problem you're encountering isn't actually related to the request method. It's related to the fact that some of the requests are getting serviced by Fiddler's pool of already-established keep-alive connections, and those connections are already established. Only when you exhaust that pool and a new connection is made is the change in the preference relevant.
To clear the Keep-Alive pool, you can either hit CTRL+X (to clear the Session list as well) or you can use the scripting engine to call FiddlerApplication.oProxy.PurgeServerPipePool();