Numista Rarity Index bug

Every time a coin is added to the collection of a user, the rarity index of that coin lowers after a couple of days max. As of this month, my most recent additions seem to not lower the rarity index. Could this be a small bug or just an aftereffect of recent updates to the website?🤷  

Not every time an item is added, only every time the threshold of the number of users having an item in their collection is reached. Many of the new additions are either rare, obscure, expensive or a combination. But a few are also recent and relatively common but nobody owns them.