Can't open shulker box facing downwards when there is a slab underneath

When a shulker box facing upward (the left shulker box in the picture) and there is a slab above - I can open the shulker box (That is ok).

But when a shulker box facing downward (the right shulker box in the picture) and there is a slab underneath - I can't open the shulker box (That isn't ok).