Steps to reproduce:
- Open the attached world 1.16 Lighting Repro.mcworld.
- /tp 290 73 233 90 and notice that all the armor stands appear lit up.
- Fly or teleport to coordinates 490, 68, 233.
- Either place a light source there or remove the one that's already there. (The point is to cause a block light recalculation in that chunk.)
- Fly or teleport back to 290, 73, 233.
The armor stands remain brightly lit because they are 2 blocks from a light source.
The armor stands are in shadows. If you place a light source near them, the light levels are recalculated. If you then removed that source, the shadow does not come back.
There is a chunk boundary between the light sources and the armor stands. When you fly 200 blocks east, you cause the edge of the ticking area to align with that chunk boundary, with the light source on the nearer side and the armor stand on the farther side. The shadow appears in the farther chunk when you cause a lighting update with this alignment in effect.
People have observed that you can fix the shadow by placing a light source near or in it, but that the fix doesn't persist. I expect that's because placing the light source in the shadow causes a different shadow to appear at the edge of the ticking area as it exists at that time. The player then travels around looking for more improper shadows, finds the new one, and place a light source to correct it. But reciprocally, that causes the original shadow to be at the edge of the new ticking area, so it returns. It ends up being a game of whack-a-mole.