Wood Variants
Racks come in twelve woods. The recipe is the same for all of them, the planks decide which one you get.
The full list
Use these names with /racks give:
acacia bamboo birch cherry crimson dark_oak jungle mangrove oak pale_oak spruce warped
Pale oak was added after the screenshots above were taken.
Racks of different woods do not stack
That is normal. Each wood is its own item.
On a server where players read different languages, two racks of the same wood can also fail to stack, because each one is named in the language of whoever received it. Set language-auto-detect to false in config.yml if you would rather every rack always stacked. See Language.
Heads show the wrong texture
The rack item is a player head, and the skin behind it is fetched from Mojang the first time a client sees one. A player who was offline at that moment sees a plain head until their game cache updates.
Reconnecting once while online is usually enough. If a head is stuck, close the game, delete the most recent folders under .minecraft/assets/skins/, and start it again.