Elytras equipped by mobs appear transparent

What I expected to happen was...:
I wanted the mob to have an elytra for map making

What actually happened was...:
It sometimes came out normal, but then others would go transparent or change from normal to transparent

Steps to Reproduce:
/summon (Whatever entity can make the elytra visible) ~ ~ ~ {ArmorItems:[{},{},{id:elytra,Count:1}]}