When copying a banner in a crafting table or your inventory if you put any random item with the banner you are copying that item will be consumed. Ex. I put my banner, empty banner, and any random item, the empty banner will become a copied banner but the random item will disappear.
Linked issues
duplicates 1
Comments 2
@@unknown No, banners cannot be copied using looms - this is still done using the crafting grid. This should be resolved as a duplicate of MC-157665.
Looms are now used to make and copy banners, please make sure you are using the latest version of the game and no mods.