I’ve had this bug for a few days now, still not fixed so I tried something. I had those Crystaline Band and Ring of Red Death, both infuse and wanted to attune them (to get the AR to be able to actually beat the content, no big deal). But they weren’t showing in the list in the mystic forge. So today I sacrificed 10 pristine relics (bye bye fractal skin, the collection is bugged too anyway) to buy a new Ring of Red Death wich I attuned and infused without any problem. Seems weird enought, the ones I had before were still not upgradable. But then I tried to equip both the old and the new Ring of Red Death, and it worked in spite of the Unique caracteristic they have.
TL;DR : Rings that can be attuned are a totally new item with the same name as the old ones wich can’t be upgraded
