Mew can only be transferred ONE TIME, into one copy of Pokémon: Lets Go, Pikachu! or Pokémon: Lets Go, Eevee!.
Once it has been transferred to a game, it will no longer be available on the Poké Ball Plus to transfer again to a different game.
Deleting a save file with Mew in it will also delete Mew. If you have deleted a game with Mew, you will not be able to get Mew a second time on a new game from the same Poké Ball Plus.
[Source: nintendosoup.com ]