This "special rule" is related to the core setting of the game - the island mechanism. Nintendo has made it clear that the island data of each Switch console is independent. In short, each console corresponds to an exclusive island. Players cannot create new islands through "reset", nor can they transfer islands to other Switches when the console is damaged or shown to friends - island data is always bound to the console and user account. But now the good news is coming: Nintendo has confirmed that players can completely migrate island data to Nintendo Switch 2!
Nintendo technical director Takuhiro Doda explained to Polygon: "It should be noted that during system migration, all archive data will remain on the original Switch console and will also be transferred to Switch2. However, the "Animal Crossing" island is a physical migration, which means that it will be completely transferred from the original Switch to the new console."
While players can transfer island data, it's currently unclear what will happen in Assemble! Will "Animal Crossing: New Horizons" launch an enhanced version on Nintendo Switch 2?