Nether portals do not cull against other nether portals with custom models

Discovered while testing MC-270233.

The bug

If nether portal blocks are assigned custom model files, they do not cull against adjacent nether portal blocks of the same type where they should. They do, however, correctly cull against solid blocks, e.g. the portal frame.

How to reproduce

1. Download and apply the attached resource pack
2. Observe a nether portal

Expected results

The portal should look like you filled a portal frame with pink stained glass, but with particles.

Actual results

It looks different from if you were to fill the frame with glass, since you can see the inner faces of the "glass blocks" even though you shouldn't.