End Portals generate incorrectly under certain circumstances

What I expected to happen was...:
End Portals should generate the black portal part when Eyes of Ender are attached

What actually happened was...:
The black portal part either does not generate or generate in the wrong place, according to your facing when you were placing them

Steps to Reproduce:
1. Build a portal
2. Attach the eyes
3. The portal may not generate correctly