I have the same issue, and I think I know how it glitched. I currently have the Ferelden, the Qunari, the Chasind and the Inquisition thrones with both of their accessories, but my collection states that I'm at 11/21 instead of 12/21.
I loaded previous saves to understand where it might come from, and it's because I never got the first Ferelden throne accessory, even though when I put it in display I definitely have it. I loaded my game just before unlocking Cresswood, and no matter how many times I try, when doing Cresswood ressources operations, I only get the second Ferelden throne accessory.
