Just noticed this myself. The most recent article in NetNewsWire is showing as "Apple Continues to Diversify Suppliers Ahead of iPhone 14 Launch", but in the 4 hours since then, "Samsung Claims Foldable Phones Are Going Mainstream" and "Apple Adds 'Test Alerts' Toggle to iPhone Just in Case You Really Want to Receive Test Emergency Alerts" have been posted to the website but still aren't showing up in NetNewsWire.
Hopefully just a temporary caching issue.. otherwise it's defeating the point of RSS.
Edit, 2h20m later: okay, both of the above articles are now showing up in NetNewsWire.