Rivens bought from Palladino didn't appear in my inventory

Hello @IdentifiedStatue! If you sort by: recent you should be able to see your new Rivens mods (possibly in a stack), you either need to activate them in the modding station or equip them on a weapon to be able to see the challenge. More information can be found in the Update 32 Patch Notes:

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RIVEN CAPACITY SYSTEM CHANGES

Those familiar with Rivens may be aware that they come in two states: Veiled and Unveiled. Both of these kinds of Rivens take up Riven Slots and can be equipped to weapons. 

After many years in the case of Riven collectors vs. database restrictions, we've added a third state to Rivens that allows them to be stackable and not take up Riven Slots. All newly acquired Rivens will be in this indistinct state, identified by ‘???’ in place of ‘Veiled’ in their Mod label and a new Mod Icon, , meaning you will be able to continue to earn Rivens as rewards beyond your Riven Slot capacity. 

In this new, these Mods will not show Riven Challenges or take up a Riven Slot until they are equipped onto a weapon. The Mod description for this new Mod type explains this mechanic as well.

Here is a breakdown of the three Riven states for clarity:

‘???’ Rivens

• All new Rivens obtained via Rewards, Palladino, etc.
• Stackable and will not take up Riven Slots
• Riven Type (Archgun, Rifle, etc.) is revealed, but not the Challenge
• Tradeable

Veiled

• What an Indistinct Riven becomes once equipped to a Weapon
• Challenge Revealed
• Takes up Riven Slots
• Tradeable

Unveiled

• Challenge Completed
• Stats are revealed and can be cycled via Kuva
• Tradeable

Since the only number we have to track with this new Riven state is the quantity owned, the database load is greatly diminished, and you can keep as many as you want! Only once a mod of this type is equipped onto a weapon does the challenge reveal itself, making the Riven apply to your account cap. This is a particularly useful change for Sorties, as you will no longer be locked out of playing them if you are at your maximum Riven cap. 

A confirmation screen will appear after attempting to equip the Riven, and will display the amount of Riven slots you have available before you commit to equipping it and using up a slot. 

Acquisition of new Riven mods from Vendors and other reward drops now give Indistinct Rivens.