Beacon beam appears dark/black at high Y-levels

I built a beacon in the end near the bottom of the world. The beam correctly appears white near the beacon block but around Y-level 100-120 it incorrectly darkens, and appears black at higher altitudes. This also happens in the overworld at night.

• Placing a stained glass block in the darkened/black part of the beam makes the entire beam white below the block, and the appropriate color above the block.

The screenshots were taken on the surface of the main end island, at about Y-level 190. The beacon was formed by a 3x3 square of iron blocks at block level 0 or 1.