I have no clue,this router must be "special?".
Years ago routers used to have a 5 port switch chip in them that runs the lan ports. It would have 4 ports connected to the lan ports and the fifth going to the router cpu chip. More modern routers now have this switch chip on the same silicon as the cpu but it functions the same.
In most routers you can't filter traffic going lan/lan. These switch chips are designed to be fast and use asic to switch traffic between the ports. Anything like say traffic filtering that would slow things down has been eliminated. The only way to filter traffic would be if the traffic would go up the connection to the CPU chip.
If you are using 2 lan port, one to the main router and the second to your PC the traffic should always stay on the switch fabric.
Something must not function in that way on your router.