To reproduce:

• Select a skin from the Sonic the Hedgehog pack (in my case, I chose Silver the Hedgehog).
• Log in to a multiplayer server (in this case, Mineplex).
• Check your character's display model.

What should happen? The skin displays normally.

What does happen? The skin instead displays as a standard Steve model with the Silver texture file applied over it.

Included is a screenshot of my player model with the skin equipped, rendering incorrectly.