Coinciding with the release of 2.5.4.0, there have been a number of otherwise functional hubs reporting as private blocks in both clients and crawlers.
I had simply added a filter locally and waited for other reports, but apparently some others have noticed over the past month too, without anything coming of it.
This is just a placeholder post for the fomation of a formal bug, perhaps related to UPnP changes. (r8775, r8778) (Commit range r8636-r8900)
Edit: It has been suggested that CNeighbour::m_sAddress is set with the correct address while the CNeighbour::m_pHost.sin_addr when later used for responses is sometimes incorrect.