Your question was based upon a false premise.
There is no such thing as a “PR7 website” (or domain). Google is a search engine, not a website directory. And like all search engines, they index individual pages, not websites. Google does not assign PR to websites, they assign PR to individual web pages.
A page’s PR is not influenced by the domain name where it is placed, instead, it is influenced by the pages that link to it, and the PR will be exactly the same regardless of which domain it is on if it has the exact same inbound links.
Your pages were likely ranking initially based on QDF (Query Deserves Freshness) and as your freshness wore off your rankings sank to their truly earned ranking.
This is not a new change in the algorithm, it exactly as it has worked for many years.