Okay, you didn't specify, so I tried both add-on and NFS. Both allow me to watch an in-flight recording, jump forwards and backwards, and use fast forward and rewind. As I said above, the only thing that goes wrong is that, when I get to the point that was the end of the file when I started (even if it isn't now - it's an ongoing recording, after all), the playback stops and I need to start it again.
Are you using mkv by any chance? It could be that the indexing isn't written properly until the file is closed, something like that. Try .ts, which is, by its nature, a stream format so doesn't need any knowledge of things yet to come.