I'm building a mob spawner, but the majority of the spawning area is not lit properly. F3 Says that the light level is 15 (sky), and this makes no sense. The roof of the spawner is entirely made of half slabs. As the picture shows, the lighting is correct under some of the slabs but not all of them.
Environment
OS: Windows 10 Home
Java Information (according to the Command Prompt):
java version "10.0.2" 2018-07-17
Java(TM) SE Runtime Environment 18.3 (build 10.0.2+13)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10.0.2+13, mixed mode)
I also have JDK 8 installed as shown in the screenshot.
Duplicate of MC-149518