I finally finished my Blacksmith Title Seal yesterday, my first 1 in Destiny 2: Forsaken. I was very disappointed to see that my Seal the Deal trophy failed to unlock upon completion. It's the final Destiny trophy for me to acquire, and I would really like to have it. This is broken right? This can't be working as intended.
Did you make sure to equip/activate the seal. If so, try going to the in-game settings menu, go to log out, then select switch profile (above/below the switch character option). Once logged back in, the trophy should proc.