plex.tv cookies may persist, which will log you in automatically into the forums.
But forums.plex.tv cookies may not persist, which do store things like the above.
I think I found it. It’s Prevent cross-site tracking ( a setting in Safari) which is weird because plex.tv - my login - persists, forums.plex.tv also persist but apparently not status.plex.tv which is where the popup comes from if I’m correct.
Seems to work disabling Prevent cross-site tracking.