I believe that plastic sockets was inpopular not because of the grounding issues, but for their poor mechanical strength. These metalic sockets are almost indestructable compared to those plastic ones. At least I haven't had any problems with them compared to plastic.
Did you try to check continuity between inner ground pin of the rj45 and screw hole?
I am assuming you are talking about small metal ground clips on the inside of the plastic connector. If so, then yes that is where we checked continuity from, but it was hard to take a picture. The clips on the inside are part of the metal that is soldered to the PCB (where I am checking from in the picutre), and continuity is good from the solder to the clips on the inside so it doesn't matter. We also checked the RB711 from the ethernet ground to the metal radio shield and we also get no continuity.
I know many people had issue with the connectors falling apart but we have good stain relief on our cables and we were not putting any force on the connector, but the ground issue is was prompted us from discontinuing the boards use.
Of additional note, we have recieved our first batch of RB433GL boards with the plastic ethernet, and they do not exhibit this issue. Despite the plastic ethernet connector, the ethernet shield pins connect to the PCB ground plane properly.
Maybe this is an early revision, or a bad batch of RB711 boards?