That's pretty weird. I'll have look into it more in depth then.
Also, as for the brother thing, that's a leftover from a previous idea. He and Eastwall's smiths were supposed to be siblings, and there was going to be a quest where you help them meet again after 10 years. Later I scrapped it and made up the uncle thing, but forgot to remove the reference so sorry about that lol. It's gonna be fixed in the next patch (hopefully along with the mithril part)
Edit: Just loaded a post-credits save file and it still works for me no problem. You just need to have "mithril ore" in your inventory ("mysterious orb" won't do). If you're in NG+ you'll have to reach the right point in the quest for him to forge for you though, as all the quests reset to zero.