CPU render with i7-8700B and macOS 10.14.5 Public Beta 2 to 500 iterations. This issue occurred without the Public Beta as well. GPU renders of course work just fine. This scene is alas too large to attach, but any scene will manifest them after awhile. They appear faster in more complex scenes.
If you look closely the same black pixels are noticeable scattered around the image to a lesser extent. This is the scene attached.