Slot icons only show when I put as first the smithing template (image 1). When I put the armor piece first, it shows only the smithing template icon but not the material one (image 2); I found the same behavior when I put the material first (image 3).
That's because the expected items actually depend on the template used; if you input a netherite upgrade template, it expects any diamond tool or armor piece and a netherite ingot, which is reflected in the slot outlines; if you put any trim template in, only armor pieces and a lot more "consumed material" types are expected.