When you transfer Pokémon from Pokémon Bank over to Pokémon HOME, it takes the National Pokédex data from Pokémon Bank and implements it into Pokémon HOME without the need to transfer every single species and form. This is very useful for various forms you may not have to hand from previous games.
Since Unbound Hoopa reverts to confined Hoopa in any pc/storage system, the only way you could get Unbound's entry on Bank would be having Bank scan a games entries and add them to its dex. :P
The same (or similar) principal should apply to Home, but I guess something went wrong on your end. :P
When you connect to Pokémon Sword & Shield or Pokémon Let's Go, Pikachu & Eevee, it doesn't take the Pokédex data from the game but requires you to put the Pokémon into HOME. Gigantamax variants will only appear if you have the Gigantamax variant and forms that only appear in battle will appear if you have seen them.
There are only 2 possibilities with this, option 1 being that you have to reconnect to Bank with the dex and hope it finds Hoopa's form in its dex, or wait until the dlc content is released, and or Hoopa is usable in game, change it to Unbound, and hope Home is updated to register SwSh Pokedexes. :P
Unfortunately, I wouldn't be surprised if the later doesn't happen; I mean, just look at Nintendo's solution for the Switch's battery problem (granted, that article is pretty satirical, but true). Sorry for the inconvenience. :P
Hope I Helped!