Banners with patterns only render the top pattern and otherwise black if a resource pack is applied

The bug

Banners with a pattern on them render black if a resource pack is used. Additionally, only the top pattern shows up. That means that the textures' transparency is not handled properly.

To reproduce

1. Create a non-black banner with two patterns on it

2. Enable the programmer art resource pack
   → ❌ Only the top pattern shows up