I looked up fixes for this in this forum, asked in Map chat, asked in guild chat. No one has a workaround for this.
I found and interacted with all items for the achievement, but it didn't give me credit for the armor piece. But now I cannot interact with the armor piece to try again to get credit. However, I also do not get this dialog option that everyone is talking about which supposedly fixes the issue (as you can see.) I really don't want to have to wait months for a fix to get the griffon mount. (I'm having trouble attaching photos to this post. I'll try to post them as a comment. )