This is possibly (even likely) a matter of not being able to transfer cloud save data from Switch 2 to the previous generation once enhanced capabilities, such as the sharing of prebuilds and items, are applied.

For the other games with Switch 2 versions (Mario Party, Metroid, Kirby), I'm not aware of any additional functionality that would prevent a player from saving on Switch 2 then picking up for continued play on Switch 1 at a later time.

Features that are exclusive to the Switch 2 version, like quick build and item sharing, naturally can't be applied to the non-upgraded version of the game. But if Metroid (for example) benefits from enhanced resolution and frame rate on the Switch 2, there's no reason that should preclude continuation of play on a Switch 1.