Before rotation each year we refresh the 10 dual-color starter decks and ensure that all players have the cards needed to play them (so everyone always has decks they can play in the rotating formats). You're probably seeing the new cards added for that, and you can find those new decks in your Starter Decks folder.
See the "New Player Decks" section here for more info on that: https://magic.wizards.com/en/articles/archive/magic-digital/mtg-arena-renewal-rotation