Common versions of Monster Infrequents have been removed from the loot pool. As some of these items had unique appearances, a special vendor has been added to Fort Ikon that sells various common Monster Infrequents without affixes for collectors and the Illusionist.
Maybe this? Explains why they don’t sell yellow rarity MIs.
But they used to sell many copies of rares. They only get one of each now which is kind of a bummer. Now have to reroll the game over and over if I need rares for crafting. Exit/restart = content ;)?