It's not a new rule, begin reading at post 20 of this thread from 2011:
http://www.flyertalk.com/forum/ameri...traveling.html
...but bear in mind that Priority Pass is a network of independent lounges. Priority Pass's terms tell us that they are not required to accept passengers without an outbound boarding pass, but in practice if the lounge is not near capacity they will earn extra revenue if they make an exception.