This gift was given to players of Pokémon HOME, as a reward for completing the National Dex. Initially, due to issues with the detection code, players had to shift their entire living dex in their boxes in one session, in order to unlock this Mystery Gift in their Pokémon HOME. This event also marks the first time Original Color Magearna became available to players.   It appears that this gift was initially intended to be released at launch, however they weren't ready to release it;