Skylight level is always 15 under Y -2048

Skylight has the incorrect value at low Y levels.
This Y level can be reached with a custom datapack that has the bedrock very low:

Reproduction Steps

• Create a world with the attached datapack

• Go into spectator and start flying down

• Observe the skylight value

Observed Results

• The skylight value suddenly becomes 15 at around Y -2048

Expected Result

• Skylight would remain 0