good day dear
in many designs of the triple offset valves i have seen bi directional seals are used but regarding the reason i cant see that it is due to bi- dirrectional flow . due to that the seat arrangement won't be symmetric to allow the offset seating snd movement of the valve disc ,so i think that it is used to prevent erosion on the rear of the valve seat which is partially a part of the valve body and sometimes welded..
regards