Firstly this bug only occurs when the item is held it doesn't affect the GUI item texture just what is seen in your hand.

When holding enchanted leather armor the "shimmer" only appears on the part of the armor that gets recolored by dyes, this still affects the item when it is dyed.

(unsure if it is in any earlier version prior to Snapshot 12w49a only noticed while playing in that version)

(the way i understand it the shimmer is only being applied to the mask, the separate image used for coloration, and needs to be applied to the actual image as well when in hand)