Right, maybe there's something I'm missing here, but I'm running Inventorio in a 1.19.2 Forge modpack of my own creation.
Maybe I'm misunderstanding things, but I assumed that the 4 slots across from the Armor were some kind of "cosmetic slot" or something.
However, when I placed my armor into the slot, it vanished entirely. I tried this with some tools and a few wooden blocks, but none of them remained, and the 4 slots are just acting like a Trash slot, instantly vanishing forever.

Here's an image of the slots that I'm talking about, specifically.