add this line into 2605994347/resource/set/vision/vision_fields.inc :

{"tank_rear-mg"
{uncover 20}
{h_fov 50}
{v_fov 50}
("spotting" r(0.85))
}

just underneath this line

{"tank_hull-mg"
{uncover 20}
{h_fov 30}
{v_fov 30}
("spotting" r(0.85))
}

fixed the issue for me