Sudden shadow/awkward lighting

Even with smooth lighting on, there is a very abrupt shadow rather than a gradient. This creates a very dark shadow, sometimes even darker than in the provided screenshots; affected blocks can even be pitch black as a result, though not the case here. Despite a massive overhang (cave) some parts remained lit and only a subsection of the cave was affected by this glitch. It was fixed by breaking a block, the diamond in one screenshot replacing the one that was broken for reference.