Whenever I try to move the lower spine, the part of the chest won’t follow the bone, and when I check the weight paints, the mesh appears red in the right areas. There isn’t any other bone controlling this part of the mesh, and it seems like blender is mad right now.
If you really want them to, they can be parented with “KEEP OFFSET”. That way they’ll follow the pelvis, but not be directly attached to it, so they can still be moved separately.