position-area in a :host rule positions the host, which lives in the outer tree. But the generated --pa-value-* mapping rule is inserted into the shadow root, where it can't match the host, so the values stay undefined and the host is never positioned.
position-areain a:hostrule positions the host, which lives in the outer tree. But the generated--pa-value-*mapping rule is inserted into the shadow root, where it can't match the host, so the values stay undefined and the host is never positioned.