The bug

When adding cullface to the tops of bottoms of glass panes, nothing happens. It is yet to be discovered if this affects any other blocks.

How to reproduce

1. Create a resourcepack that tries to add cullface to the tops or bottoms of glass panes
2. Open the game
3. Place a glass pane with a full opaque block above or below
4. Go into spectator mode and view the top or bottom of the glass pane from inside that block and notice that they're still there