Sign crafting recipes of various wood types are not grouped

The bug

Unlike the crafting recipes for other wooden items (i.e. stairs, slabs, etc.), the crafting recipes for signs in the recipe book are not grouped together as a single icon that periodically changes between all the wood types.

To reproduce

1. /recipe give @p *
   

2. /clear

3. Put a stack of sticks and a stack of each type of plank into your inventory.
4. Open a crafting table. Open the recipe book. Check the box in the upper right corner to only show currently craftable recipes.
5. Type "stair" into the search bar. See that the recipes are grouped as a single icon.
6. Type "sign" into the search bar. See that the recipes are not grouped as a single icon.