If I place a slab on the bottom side on a glowstone or an other light source, the upper side will be full of light and the rest of the room will be dark. In MC 1.12.2 the room gets lighted up, as it also shout in MC 18w20c.
MC-129926
Light glitch with slabs
Resolved
Fixed
0
(Unassigned)
Community Consensus
Minecraft 18w20c, Minecraft 1.13.1, Minecraft 1.13.2-pre2
Minecraft 18w46a
Retrieved
Confirmed for 1.13.1.
Relates to MC-92.