I tried also that but in the end I had to return to one old saves that doesn´t have the issue with John.
Now the one with the materials were also fixed returning to my old save but it came back when I killed a chicken in the inventory to gather chicken meat. Idk what to do cuz I advanced a lot with this save and I don´t have any other save to go back.
Are you saying that John quit working on armor, after you killed a chicken?
Those things aren't connected, especially since you can't see John from Penny's chicken coop or at a trap in the wild.
Did you just happen to kill the chicken around a clock sync time (always happens at midnight, can happen at other times) and got the alert that John was out of Iron? If so, don't trust the alert, trust what you see in John's shop. If he has stacks of Iron bars, you don't have to worry about the baskets of iron ore. If he doesn't have pyramids of iron bars, and you want merc armor, you have to get him the bars.
You can test whether or not John really has merc armor supplies by clicking on his anvil and choosing "assist John", if you can he is making your armor. If you can't, you have to figure out why. I promise you it has nothing to do with chickens.
It will have something to do with the Kings order, or having the mine opened and guarded.