Wither Skeletons don't spawn

^{Update by [Mojang] Mega_Spud (Jay Wells):}
Summary:
Wither Skeletons do not appear to be spawning in Nether Fortresses in the 1.11.0.x beta version.

Steps to Reproduce:

1. Travel to a Nether Fortress
2. Ensure the difficulty is set to Easy or higher
3. Set a repeating command block to kill all mobs except Wither Skeletons:
   kill @e[type=!wither_skeleton, type=!player]
4. Set another repeating command block to detect Wither Skeletons:
   /testfor @e[type=wither_skeleton]
5. Wait for Wither Skeletons to Spawn!

Observed Results:
Wither Skeletons are not spawning in Nether Fortresses.

Expected Results:
Wither Skeletons should spawn frequently in Nether Fortresses.

Screenshots/Videos attached: Yes

Notes:

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Original Description:

Wither Skeletons don't spawn in nether fortresses.