Resolved: BiS unexpected results for changing Unity & Legendary slots

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For this Shadow Priest character of mine, BiS is recommending Shadowflame Prism in the head slot, which is good to see. However, when I force Unity to be the head slot instead, to accommodate a BiS Holy setup without crafting two Unity pieces, BiS suggests a different legendary altogether.

What is even more surprising is when I force Shadowflame Prism in the hands slot, the DPS increase drops by over 30%! In other words, all that changes from BiS’s original recommendation was switching up the Unity and Shadowflame Prism slots, and this led to a decrease of 30% DPS. This really shocked me and I’ve never seen it happening before.

Is someone able to explain why this is happening and confirm that it is accurate or working as expected, please?

And are there any ideas to help me avoid crafting more than one Unity while still getting acceptable results in both specs Shadow and Holy?

Update: After locking Unity in other slots, I saw the pattern and it dawned on me. This is another issue of BiS not being able to complete a 2pc or 4pc (4pc in this case). And in this case, there’s the very unfortunate situation that Shadowflame Prism’s two slots are both tier slots, so if Unity also occupies a tier slot, the 4pc is gone, leading to this huge decrease in DPS potential.

And in this case, the Waist slot is the best slot for Unity to achieve the best balance between 3 setups, Holy Raiding, Holy M+, and Shadow M+. I had to lock Unity in each slot for all 3 setups and register the percentage increases in a spreadsheet then sort them from highest to lowest to conclude this, though. It would be awesome if the AMR team adds a feature that answers this question for us without this manual work:

Which Unity slot is the best for balancing the ticked setups?