The bug
Blocks that emit light which are placed or activate in lazy chunks (chunks were only redstone is processed) they will not cause a lighting update.
How to reproduce
Set the time to night
Use the following command, replace
[x]
with:Minimum(8, renderDistance) * 16
(For example your render distance is 7:8 * 16 = 128
)/setblock ~[x] ~ ~ sea_lantern
Go to the position where the sea lantern was placed
→ It should emit no light
Confirmed in 16w41a and 16w42a