I've done two external builds this week, and in both cases the external testers aren't getting Push Notifications or emails that a new build is available. If they open the TestFlight app they see the new build. Obviously I'm talking about people that have everything turned on.
So I guess this is more of a heads up to anybody confused by what's hapening. Just tell your testers to open the TF app if it comes up. I filed bug #31626719 about this.