I made arrowheads from scrap metal and now they remain in my inventory no matter how many stacks of complete arrows (fletching, shaft, arrowhead) I fabricate.
Just to get this out of the way, you didn’t happen to have a pile of arrowheads nearby (not including the one in your inventory), did you?
I was using the only stack of arrowheads which I was storing in my inventory. I must have crafted at least a hundred or more arrows now and I only ever need to craft a shaft and fletching, no arrowhead needed except the one that was not consumed from the last batch.