This sounds like one of the new Personal items.
In the loot filter two new sections were added, Experimental and Personal. Experimental is for all the experimental affixes that can drop on items after killing an Exiled Mage. Personal is for all the personal affixes that can be on items looted from One-Shot Caches. You’ll want to create a loot filter rule to show or recolour these affixes so that you don’t miss these items that your filter may currently be hiding.