After some testing yesterday and noticing something like this was going on, and after reading several other posts on this today, I decided to do some digging. I believe the answer is that one afflock's Haunt either overwrites or cancels out the other afflock's Haunt.
Follow with me here:
From the log:
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02:56'44.625 Nosfaratu Haunt hits Patchwerk for 1942 Shadow. (191 Resisted)
02:56'44.625 Luxury gains 50 Mana from Maidensorrow Replenishment.
02:56'44.625 Nosfaratu gains 46 Mana from Maidensorrow Replenishment.
02:56'44.625 Patchwerk suffers 993 Fire damage from Nosfaratu Immolate. (74 Resisted)
02:56'44.765 Luxury's Unleashed Rage is refreshed.
02:56'44.765 Nosfaratu's Unleashed Rage is refreshed.
02:56'44.765 Luxury's Unleashed Rage is refreshed.
02:56'44.765 Nosfaratu's Unleashed Rage is refreshed.
02:56'44.828 Patchwerk suffers 1142 Fire damage from Luxury Immolate. (102 Resisted)
02:56'44.828 Patchwerk suffers 907 Shadow damage from Luxury Siphon Life.
02:56'45.125 Luxury's Improved Icy Talons is refreshed.
02:56'45.140 Patchwerk Haunt heals Nosfaratu for 2058. (2058 Overheal)
02:56'45.140 Luxury gains Demonic Soul.
02:56'45.140 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'45.140 Luxury's Demonic Soul is refreshed.
02:56'45.140 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'45.140 Luxury Siphon Life heals Luxury for 961. (961 Overheal)
02:56'45.187 Patchwerk suffers 1727 Shadow damage from Nosfaratu Corruption.
02:56'45.218 Luxury's Unleashed Rage is refreshed.
02:56'45.218 Nosfaratu's Unleashed Rage is refreshed.
02:56'45.218 Luxury gains Ferocious Inspiration.
02:56'45.218 Nosfaratu gains Ferocious Inspiration.
02:56'45.218 Patchwerk Haunt heals Luxury for 1917. (1917 Overheal)
02:56'45.218 Patchwerk suffers 772 Shadow damage from Luxury Curse of Agony. (69 Resisted)
02:56'45.218 Luxury Pandemic hits Patchwerk for 2252 Shadow.
02:56'45.218 Nosfaratu gains 46 Mana from Maidensorrow Replenishment.
02:56'45.218 Luxury gains 50 Mana from Maidensorrow Replenishment.
02:56'45.625 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'45.671 Patchwerk suffers 808 Shadow damage from Nosfaratu Curse of Agony.
02:56'45.671 Patchwerk is afflicted by Haunt.
02:56'45.671 Luxury Haunt hits Patchwerk for 2711 Shadow.
02:56'45.671 Patchwerk suffers 1722 Shadow damage from Luxury Unstable Affliction.
02:56'45.671 Nosfaratu's Rampage is refreshed.
02:56'45.671 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'45.671 Luxury gains 578 health from Pretend Judgement of Light. (578 Overheal)
02:56'45.671 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'45.671 Patchwerk suffers 641 Shadow damage from Nosfaratu Siphon Life. (57 Resisted)
02:56'45.671 Nosfaratu Shadow Bolt hits Patchwerk for 3367 Shadow. (331 Resisted)
02:56'45.688 Patchwerk suffers 2684 Shadow damage from Luxury Corruption.
02:56'45.703 Luxury gains 50 Mana from Maidensorrow Replenishment.
02:56'45.703 Nosfaratu gains 46 Mana from Maidensorrow Replenishment.
02:56'45.703 Patchwerk suffers 1152 Shadow damage from Nosfaratu Unstable Affliction.
02:56'46.296 Luxury gains Ferocious Inspiration.
02:56'46.296 Nosfaratu gains Ferocious Inspiration.
02:56'46.296 Luxury gains Ferocious Inspiration.
02:56'46.296 Nosfaratu gains Ferocious Inspiration.
02:56'46.296 Luxury's Ferocious Inspiration is refreshed.
02:56'46.296 Nosfaratu's Ferocious Inspiration is refreshed.
02:56'46.296 Nosfaratu Siphon Life heals Nosfaratu for 679. (679 Overheal)
02:56'46.296 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'46.296 Luxury's Molten Core is refreshed.
02:56'46.375 Nosfaratu's Rampage is refreshed.
02:56'46.968 Patchwerk suffers 1061 Shadow damage from Luxury Curse of Agony. (79 Resisted)
02:56'47.218 Luxury's Unleashed Rage is refreshed.
02:56'47.218 Nosfaratu's Unleashed Rage is refreshed.
02:56'47.281 Luxury's Unleashed Rage is refreshed.
02:56'47.281 Nosfaratu's Unleashed Rage is refreshed.
02:56'47.281 Nosfaratu's Rampage is refreshed.
02:56'47.281 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'47.281 Luxury gains 50 Mana from Maidensorrow Replenishment.
02:56'47.281 Nosfaratu gains 46 Mana from Maidensorrow Replenishment.
02:56'47.281 Patchwerk suffers 827 Fire damage from Nosfaratu Immolate. (74 Resisted)
02:56'47.281 Patchwerk suffers 1523 Fire damage from Luxury Immolate.
02:56'47.281 Patchwerk suffers 1088 Shadow damage from Luxury Siphon Life.
02:56'47.656 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'47.656 Luxury's Illustration of the Dragon Soul is refreshed.
02:56'47.656 Luxury Siphon Life heals Luxury for 1153. (1153 Overheal)
02:56'47.656 Patchwerk suffers 674 Shadow damage from Nosfaratu Curse of Agony.
02:56'47.890 Patchwerk suffers 1439 Shadow damage from Nosfaratu Corruption.
So, as you can see from the logs. After a haunt is casted, Nosferatu's corruption ticks for 1727. After Luxury casts Haunt, his corruption drops to only 1439.
So, follow with me on the math. To start, we need to find out what his corruption would be doing without modifiers. With 1727 * 6 ticks = 10362, we have to solve for X. X represents base damage plus spell power bonus added/already calculated.
X * 1.30 (Shadow Mastery 15% + Improved Corruption 10% + Contagion 5%) * 1.03 Malediction * 1.03 Ferocious Inspiration * 1.13 Earth and Moon * 1.20 Haunt * 1.10 Shadow Embrace = 10362.
When I solve for X I get 5037.
Now, after Luxury casts haunt, Nosfaratu's corruption drops to 1439 damage per tick, or 1436 * 6 ticks = 8634.
Now, take the above equation, and remove haunt:
X = 5037
5037 * 1.30 (Shadow Mastery 15% + Improved Corruption 10% + Contagion 5%) * 1.03 Malediction * 1.03 Ferocious Inspiration * 1.13 Earth and Moon * 1.10 Shadow Embrace = 8635.
I get the same result if I do the math for immolate and CoA as well.
I do believe we have found the bug. Math geeks have at it. Let me know if I've made any mistakes. As we know, DoT "buffs" are calculated at the time of casting, and since he doesn't recast the DoTs here where we're looking in the combat log, that part would not cause this. The only other thing could be debuffs on the boss itself, but I see no debuff changes on the boss in the combat log for the time period we're looking at here.