they are two totally distinct metals with nearly similar name ... well except that you need mithril to make mithral ... mithral is kinda the steel to the mithril iron :x
edit: also now thinking of it, moods never look for a certain metal, they just want metal bars of *any* kind so your bug report seems to be kinda suspiously
the only thing would be mandates, but if they state they want mithril stuff made and you only make something from mithral it wont work. of course, just as it wont work to make steel stuff than they want iron
"BalancedIvory. A mithril sock"
That was the artifact I was able to save when I finally realized that the Mithril was being made into Mithral bars. I can respect the suspicion, as I did not even think of it, until just now, that it was strange that the Metalsmith was wanting Mithril, and not any other type of metal. If it comes to it, I will check my back-up saves to see if he ever called for Mithril, but I can assure you that he did. I do not know if it was because of the introduction of the mod, altered raws, or what, but it did. I had gold, copper, silver, lead, and hundreds of other metals, but the damned smith wanted Mithril, and Body parts, thinking of it.
Still, that seems rather redundant to call the second metal Mithral. It was highly confusing, and I didn't realize it until I received an announcement saying "Cancels make Mithral Bars: Needs Mithril bars" or the like. That, and the similarity of their names is what caused me to think that one was simply an accidental addition and/or typo.