So this has been a bug since the launch last Friday, but I wanted to give it its own post so that hopefully ANET can definitely be aware of it.
Certain Ascended Rings simply are not showing up in the Mystic Forge to attune them. It seems to be a case of rings obtained from certain methods (pristines vs. reward chest vs. infused in forge, etc.), while the same named ring obtained some other way will work.
Here is a screenshot of both my my rings in my inventory, but not in the forge for proof: https://i.imgur.com/QvAXjet.jpg
Thanks, and hopefully this can get fixed soon so I can progress to the higher fractals!