Lava doesn't decrease BlockSkyLight Level, while water does that. Because of the fact that you cannot see through Lava this is very weird. In the screenshot you can see the player fully submerged in lava, but apparently still in clear view of the sky (full skylight)
Water does decrease skylight 3 per block.
MC-106387
Lava doesn't decrease SKY light
Resolved
Fixed
1
(Unassigned)
Confirmed
Minecraft 1.10.2, Minecraft 16w32a, Minecraft 16w32b, Minecraft 16w33a, Minecraft 1.11, Minecraft 1.12.2
Minecraft 18w07c
Retrieved
Affects 16w33a