That is something that is dependant on the add-on your using and not Kodi or OSMC. If you wanted that feature added on to the Plex for Kodi add-on you would have to request it from Plex themselves.
Thanks. It was during our investigation of my other issue (that you resolved – thanks again) that I saw the "Do you want to resume from 16:23?” dialog within Plex, within Kodi, within OSMC. I saw it a couple times in passing and wanted to follow up to see if it could be made permanent.
The only thing I think you can do is ask over on the Plex forum. I don’t think this is because of any kind of a setup or settings thing that we would have any control over.