Abyss %Rarity mod not dropping?

Hello.

This mod on the Minor Idol does not seem to be dropping in Softcore Phrecia.

(300–350)% increased Rarity of Items Dropped by Abyssal Troves and Stygian Spires in your Maps.

There are 0 listed on the trade website for Softcore Phrecia. Including online/offline. However, you can find some listed in Ruthless Phrecia. Is this a bug? Or is it a ruthless only affix?

Note: The following modifiers only appear in Ruthless mode:

of the Abyss - (300-350)% increased Rarity of Items Dropped by Abyssal Troves and Stygian Spires in your Maps (Suffix)