In the attached world if you place wither roses on the netherrack under the glass panes then zero mobs spawn in those spawn points. If you remove the wither roses then any mob can spawn. Expected behavior based on the wiki is that wither skeletons will spawn bot not any other mobs. Current behavior, no wither skeletons spawn unless the roses are removed. 

Quote from wiki "Note: In 1.16, the only mob that can spawn on wither roses are Wither Skeletons." https://minecraft.gamepedia.com/Tutorials/Wither_skeleton_farming

Unfortunately I can't attach my world because it is 165MB and exceeds the limit. If any of the mods wants me to email them the world download or has a suggestion, I am happy to provide the world.