This has come up in a few topics but seems to be a separate and perhaps universal problem. It seems that when the Plex DLNA server is asked to transcode (including direct streaming), all of the seek functions drop out and trying to navigate within the stream dumps you back at the beginning.
All of the solutions that I've found seem to involve forcing Direct Play, which isn't always an option (eg if your device just isn't capable, or if you want subtitles etc). I'm not sure if these functions have ever worked using DLNA (they work fine using Plex clients) or if that's just a limitation inherent in DLNA vs HLS?
Anyway I don't think we've heard from the developers on this issue and it would be great to get some feedback.