Sea lanterns are absorbing whatever skylight value they are exposed to and give off that amount of skylight in addition to their normal block light.

To reproduce:
1) Place a sea lantern inside a building with sky light coming in.
2) Seal all possible areas where sky light could get in to the building.
3) Hit F3, observe the sky light value is not zero near the sea lanterns, despite the building being completely sealed off from all sky light.

Breaking and replacing the sea lanterns while the natural sky light is blocked off entirely is the only way to avoid this bug. If natural sky light is later allowed back in, the sea lanterns will absorb it yet again and have to be broken once sky light is blocked.

placed_with_no_skylight.png shows the result of placing the lantern after sealing the room completely.
placed_with_skylight.png shows the result of placing the sea lantern first and then sealing the room completely.