Good catch. Bug could be either or both Apple or vendor +/- the VPN implementation and if it also uses Network Extension Framework filters or whatever. The BSD-derived source code in question in netinet hasn't really changed appreciably in ages. Could also be an implied "6to4" bridge that the VPN sets up that screws things up. With the VPN running, go go System Preferences...Network...and you should be able to visually identify if there's a virtual "port" or relay.