Hmmmm, if they are there now, why are they not on original SA?.
I suspect that they have changed the way they feed the metal into the SA. I would guess that it wasn't fed in that area to begin with, and they have left part of the gating to strengthen an area that has failed before. As the aluminum cools, that area is the largest area, so it will stay liquid longest as the SA sets up. When the metal cools and solidifies, it shrinks and those "metal plates" as you called them stay liquid long enough to continue feeding into the shrinkage area.
Apologies if I offended anyone, it was not my intent. I'm here to learn, make friendships, and help where I can. This is one area I really know about. I'm senior tool designer for a prototype and low production run sand cast factory. I've been working at my current position here for 21 years, and at another place for 6 years before that. I was a manual machinist for 11 years prior to designing tools. It's what I've made my living by all my adult life... making and machining castings like the SA, transmission housings, transfer cases and covers and so on. (And yes, I'm old as $#!t )