I know that Critical Successes can happen in the Forge, but I just had a Critical Success for an affix that gave me an extra upgrade to the same affix which I didn’t have enough of. I had only one Elemental Damage Over Time affix and when I Upgraded it I got a CS for Elemental Damage Over Time.
I can’t imagine it’s supposed to give you an upgrade to an affix that you don’t have anymore in your inventory. If that’s intended then, great, but if not then you might want to take a look at it.