Describe the bug
The objects do not appear when I place them on my character.

• Artifacts: [1.2.3]
• Forge: [1.12.2]

It looks like you're using a mod that changes how the player model is rendered, which is likely what is causing this issue.
Artifacts is no longer being actively maintained for Minecraft 1.12, so you'll have to either remove the mod that is causing this or play without cosmetics.