We needed to be clear when and how Item Upgrades are unlocked. We’ve changed the Materials Tag to be a bit more clear as to our intent:
Materials are found by Defeating Combatants and drawing the Monster Loot card in the Combatant Loot deck. So long as Card with the MATERIAL Tag is on a character in the Party or in the Party Stash, all Item Upgrades associated with that Material will then be available for purchase at all Shop and Train Phases. See Item Upgrades Pg. 28.”
This update was propagated to the Monster Loot section as well.