Light Level in F3 not correct on slabs, stairs, farmland, and grass path

When standing on a bottom slab (i.e. not a full block) the light level on the F3 screen will be 0 (0 sky, 0 block) even when standing next to a block. This also applies to the bottom step of the stairs.

As far as I know only slabs and stairs have this problem. Daylight sensors, chests, repeaters don't seem to be affected, but I haven't tested this extensively.