i’m not sure why it would do this, i’ve never had any issues with watching plex while the internet is down (in fact that was one of my original uses for it, to have movies and tv in a building without internet). I don’t have it turned on but I do know you can go into server settings -> network and set a list of IPs/subnets that can access without any authorization at all. That lets you use plex without even having a plex account afaik.