Pokémon HOME's Newest Feature Lets You Change Your Pokémon's Moves

@Musharna
Yeah, starting with Sword/Shield, Pokémon now remember a history of learned moves and can be re-learned at any time at no expense. No needing to re-visit a move tutor, or use a single-use TM or TR.

They were also mindful of Pokémon that once evolved, can no longer learn moves from a prior evolution. This particular affected Pokémon that evolve using an item such as Arcanine or Raichu.

There are still Pokémon that cannot learn a move once they've evolved, such as Breloom being unable to learn Spore if they did not learn it as a Shroomish.

@AlexanderDaniels
It's more than likely a data structure issue.
The basis for the Pokémon data has been built upon ever since generation 3, and things like keeping a history of learnt moves was introduced in generation 8, and therefore cannot be assumed when importing from prior generations.

There's also the sticky issue of moves being outright unavailable in games starting with generation 8, such as the removal of Hidden Power.
Further to that, generation 9 notoriously removed Toxic as a TM, a move that nearly any Pokémon can learn via TM. This could be part of an intentional redesign of move legality.