light level of 1 in the nether is preventing wither skeleton spawns

placing any light level even a light level 1 light block will prevent wither skeleton spawns inside a fortress 

Steps to recreate

Find a fortress
use commands to fill the area with level 1 light blocks, (or any other source of light)
observe no wither skeleton spawns
other nether mobs spawn normally

expected behavior

Wither Skeletons being a nether mob should not be affected by light level