Streaming services count concurrent streams and, increasingly, verify that devices belong to one household. When either limit is exceeded, playback stops with a message that rarely explains which device caused it.
Profiles are separate from accounts. Watch history, recommendations and downloads live in a profile, so the complaint that everything looks wrong is usually the TV defaulting to someone else's profile.
Sudden sign-outs on a TV are often deliberate: services log all devices out after a password change, and hotel or holiday sign-ins may trigger a household check when you return home.
Keeping one device as the household anchor, and signing out of hardware you no longer own, prevents most of it.