Starting this week, something either changed on Blizzard's end or with some library Rarity uses which made it so Rarity is no longer picking up on Pandaria World Bosses attempts.

Debug output from fighting Sha of Anger:
[09:53:01] Rarity: Mouse hovered over NPC with id = 0
[09:53:01] WBT: Sha of Anger is now engaged in combat!
[09:53:01] Sha of Anger yells: Your rage gives you strength!
[09:53:01] Sha of Anger yells: Yes, YES! Bring your rage to bear! Try to strike me down!
[09:53:02] Rarity: Detected UNIT_SPELLCAST_SENT for unit = player, spellID = 185123, castGUID = Cast-3-4222-870-94-185123-0016FAF8AD, target = Sha of Anger
[09:53:07] You receive item: [Cache of Sha-Touched Gold].
[09:53:08] Rarity: BAG_UPDATE
[09:53:09] Rarity: Mouse hovered over NPC with id = 60491
[09:53:09] Rarity: Mouse hovered over NPC with id = 60491
[09:53:09] Rarity: Mouse hovered over NPC with id = 60491
[09:53:12] Rarity: Mouse hovered over NPC with id = 60491
[09:53:13] Rarity: Mouse hovered over NPC with id = 60491
[09:53:23] Rarity: Detected UNIT_SPELLCAST_SENT for unit = player, spellID = 132187, castGUID = Cast-3-4222-870-94-132187-0038FAF8C2, target = nil
[09:53:23] Rarity: Currency updated (CURRENCY_DISPLAY_UPDATE)
[09:53:23] Rarity: Used coin: Elder Charm of Good Fortune
[09:53:23] Received 42 Gold, 5 Silver, 26 Copper.
[09:53:25] You loot 42 Gold, 5 Silver, 26 Copper
[09:53:26] Rarity: Mouse hovered over NPC with id = 60491

Let me know if you need more data or information.

Issue apparently solved itself yesterday, likely something Blizzard's end.