The bug

When creating a new world the small water ponds occasionally tend to be too dark. This isn't just a visual effect but a deficiency in the actual sunlight level of the air above the water, as shown in the debug output.

I'd speculate the direct sunlight probably stops at the point where the original terrain was, before the world generator carved out the pond.

Updating lighting by placing a torch seems to fix it, but reloading the world brings the wrong lighting back again. However, filling the entire hole (except for the water) with blocks up to the previous terrain height and digging it out again fixes it permanently.

How to reproduce

1. Generate a new world using the seed -4270258239336733475.
2. Fly to the coordinates -568 77 -149 (Do NOT teleport there, possibly due to the chunk population order being different, it seems not to happen when teleporting)
3. Press F3 and look at the light level. Visually it can be best seen with moody brightness.