Sign out unknown devices on iPhone

Can you please provide more information? What other devices are signed in to your iPhone? Where are you seeing this?

In general, if you open Settings on your iPhone, then tap your name, you should see the list of devices logged in to your Apple Account.

• Tap to select an unwanted device,
• Then tap Remove From Account.
• Next - and this is important - tap the Back arrow at the top, left of the screen to save your action.

I hope this answers your question.