I've built a simple trading hall (see photo) on a survival server with the villagers in a one-wide, tileable, double carpet system. I have around 50 villagers employed, with librarians, farmers, armorers, weaponsmiths, and toolsmiths. The last three are used for the iron trade, and all of them currently offer it. However, when I go to add a new armorer, it never produces the iron trade upon level up to apprentice. The probability should be 1/2 chance of having it, but I've done eleven new armorers in a row with no success. Is this a bug or just bad luck?

Edit: that number is up to 16 now. I always seem to get either bell+boots or bell+leggings for apprentice trades. I'm also going to switch to toolsmiths or weaponsmiths, since those trades are guaranteed to have iron at apprentice level.