I see there no issue at all.
* The settings what you need for crafting items are different in each level.
* There are in each level items, where crafting them is really worth doing that for profit, while others are crafted with loss, but you need the e.g. for getting techs. For generating profit it is also important, that you need not to many smelters for them.
* However smeltings bars for the creation of items is in several levels also not profitable, so you make much more profit with buying bars (at least, as long the market price does not reach the creation price)
* All of that above may vary a lot for individual players, because it depends on techs, advancements and artifacts, being active.
* Last but not least, those settings may change for each version of the game. So don't rely to much on older forum posts, telling you numbers.
So it is part of the game strategy, to find for each level the perfect solution for maximal success. For example in some cases its also much more beneficial, to buy very expensive bars from market, instead of smelting them.
This is in many situations the case, if smelting other bars gives you much more overall performance.
A calculator app is in this game your friend.