I am the owner of a Plex Server, and I have a Plex Lifetime Pass. I don’t see ads, but I have a remote user who has started seeing ads on content from my libraries. My understanding is that if the server owner has a Pass, all other users should be able to stream without ads.
Can somebody tell me how this is supposed to work, and what I can do to get rid of the ads?
Ads have nothing to do with Plex Pass. Ads only appear on Plex-supplied content.
The user you are sharing with isn’t watching the content from your library, they just think they are. They are watching a stream from the Plex ad-supportted VOD service.
Your user likely has online media sources enabled in their account settings. When they searched or selected something in your library, it was also available on the plex streaming service and they accidentally clicked that source.
Tell them to disable those in their settings if they don’t want to accidentally make that selection.