I thought all uniques literally had unique skins.

Just put on

and


but got the regular print and a spikey shield shown on my character model.

There is no skin transfer on them (why would you skin transfer a normal look to a unique to begin with) so is this just a bug/glitch or are many uniques just regular skins of the normal equivalent?

Many uniques don't have proper 3D art. Since forever they've been adding uniques to the game faster than they can make models for them. I'd say around half do, though every large patch adds more for old uniques.