With the lighting rewrite in 1.14 allowing for directional opacity, it seems that lighting as a whole just gives up if the player ascends any further than y=2048. This issue first appeared in 18w43a, a few snapshots before the directional lighting change. Above 2048, everything is dark, below -2048 appears to behave as full daylight instead.

For the horizontal axes this limit is 33,554,432, which is impossible to encounter without mods (but is incredibly annoying with mods).

Expected behaviour would be that light functions correctly above 2048 and beyond 33554432, out to the 32-bit limit (or perhaps even theoretically beyond) on all three axes, which it does in 1.13.2 and before.

If fixed, I would recommend moving the default world border out to 2 billion blocks, since with this out of the way, there should be no more game-breaking bugs out at this distance.