wait can i just deactivate my burrow? or do i have to delete it
Just answering the burrow question (until zh mentioned it, I didn't know that was part of the issue), if you have a burrow set up with a dwarf in it then the dwarf can get released by
either deselecting him from within the burro(w)-(c)itizens list
or deleting the burrow.
Exactly the same effect, except that the latter will also release any other dwarves currently assigned to that burrow, whereas the former keeps them in.
What this has to do with the material gathering, I don't know. I might use burrows to 'pursuade' a moody dwarf to pick up certain materials and not others, but I saw no sign that you'd done that, and I'm not aware (unless you've been consistently forbidding the metal bars once they've been brought to the workshop, and he's just going out to get a replacement bar, over and over, but you should have een directly involved in that happening, so should know) of any situation where the original request for bars should loop due to burrows.
(Only possibility I can think of, is that
after the workshop was claimed (perhaps before, but I'm sure you can steer them to specific workshops of-a-type with burrow assignments), you set up a burrow that did not cover the workshop. Again, you should know that you did this, and while maybe moody dwarves don't have "destination unreachable" errors I still think there'd be some other indication that this happened.)
Just going on personal experience, there.