I have never seen this. Anytime a relationship is defined in more than one connection, the originating FlowFile will be routed to one connection and clone routed to the other(s).
Perhaps you can share a some screenshots of your dataflow showing the behavior you are seeing? Screenshot of flow feeding the output port inside PG1. Screenshot of tope level interconnected Process group PG1, PG2, and PG3.
Is the input port inside PG2 connected to anything?