Were they specifically caught and registered from Let's Go or were they caught in like Pokemon Go and then transferred to Let's Go? They have to have the Let's Go mark in their summary to properly register for that Pokedex.
I mean, I understand why they did this, because they want you to actually engage with certain mechanics in the games, like actually completing the side quest in Arceus to get Spiritomb instead of just catching one in the wild in SV
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The regional dexes in Home are under the ''Games'' category for a reason. It's meant to register pokemons caught in specific games.