Caliban Darklock
Comments
I've been seeing this as well. Exiting the world and returning fixes the lighting, but when the light problem is visible, mobs can spawn at or around that area - in the first screenshot (20210302131947_1.jpg) you can see the black area in a well-lit part of the map, and in the second (20210302143331_1.jpg) you can see a pack of skeletons in the fully lit field with a creeper at the left edge. These mobs literally spawned as I watched, with the light levels as they appear here.
Running "/debug report" on a few seeds under both 1.16.5 and 21w07a, I am not seeing any dungeons (spawner with chest nearby on same Y level) near spawn on 21w07a, even when the 1.16.5 world shows several of them. Several spawners on 21w07a seem to be at the exact same X/Z locations, but 11 blocks higher on Y.
Of course it's going to be fixed, but these are snapshots and large bugs are often pushed until all the features are in.