This bug occurred to me when placing a lightsource in undeep water (approx 5-10 blocks under the sea level). When placing and removing a lightsource, sometimes the lighting level stays at 15. I have attached the world and some screenshots. As seen in the screenshots, I am under water, not very deep (screenshot 1a). Then, I place a lightsource (solid lightsources but also torches and end rods work) (screenshot 1b). When I remove the block, the lighting level stays at 15 instead of dropping back down to its original value (screenshot 1c). In screenshots 2a and 2b I have done the same thing but with and end rod. I have been able to pull this off in a lukewarm ocean and in a river, so I don't think the biome is specific. I have also attached an example where it didn't work for some reason (screenshots 3a-3c).

Unfortunately, the bug seems to be random. I haven't been able to find any sense or logic in it :/. Reloading the world doesn't help either, so it's not a visual glitch, but an actual bug.

The world download can be found here: http://www.mediafire.com/file/f2lftv2ef0qsngu/Snapshot%2018w20c%20-%20flooded%20base.zip

It's not a major bug, but I hope it gets fixed before release. You guys are doing a great job on update aquatic!