Has been happening to me as well for several months. This does not seem to map to either an Xbox One system update or a Plex update. Every 2-3 days will prompt for pin; often enough that Chrome on my phone now auto-suggests the plex.tv/link address by default when I launch it.
Read through a lot of the older responses on this thread - I do have multiple users on my Xbox but the “primary” account auto-logs in and is almost always the one used to access the Plex app. Only one Plex account is signed in to the app, though there are multiple users set up within Plex once signed in.
