Light is broken (unusually dark shadows)

There are 4 unrelated bugs having similar symptoms of dark shadows:

• This bug causes unusually dark but still gradual shadows. It can be worked around by momentarily placing a torch or other light source in the shadowed area, or by any action that would cause a lighting update such as breaking blocks above the shadow to expose it to the sky light.
• MCPE-39368 describes a bug that causes fully black shadows with sharp edges, even right next to a light source. Placing a torch does not correct this problem.
• MCPE-58182 describes light not being updated properly across chunk and sub-chunk borders when areas are reloaded. Placing a light source does correct this problem.
• MCPE-69037 describes the entire world being dark or hazy after return from the nether or end, and light source blocks being the only blocks that are lit, with no light spreading from them.