Light Overlay rendered inside the non-solid blocks even if they have blocks on top of them.

(just to see im in the blocks)

it is also happens with mud blocks, and 8 layer snow blocks

It's just how it has been implemented, i.e. which checks the renderer does by default to decide where to render the overlay. Basically the default condition to prevent the overlay from rendering is a normal full opaque cube above, and soul sand is not such a block, so it allows it to render. It also renders inside bottom slabs for example, if the block below the slab has a spawnable top surface. The main reason for this is that the "proper" collision box checks are slightly slower per each block, and the renderer needs to check quite a few blocks each time it updates.

There are several light level related options in the Generic category which allow you to control this behavior, and I will also add at least one or two more in some upcoming version.