Not sure whether this is an issue, intended behaviour, an issue with Postman or my error.
I'm logged into my site as user 1 in Chrome.
Using Postman if I fetch /users/me with the Authorization for user 2 and then call $this->auth->users in the controller, it returns user 1. If I call ResourceServer::getOwnerId() (I'm using the League OAuth provider) it returns user 2. If I logout of the website, $this->auth->users then returns user 2. So it looks (looks) as if the logged in session is overriding Shield and $this->auth.
I'm not sure in practice whether this is an issue but thought I'd mention it.