I just replaced Felsoul Leggings with another item - so I was wearing it, and learned it. Further, both Caerdon Wardrobe and Can I Mog It? See that I've learned the item. ATT says not learned, and the tooltip says that another item is missing in ATT. Here is a link to a screenshot of the tooltip: https://imgur.com/9IDKtOk

You fix this by Shift + Left clicking the header. If this is still an issue, please open a new ticket.