That player's discard pile

[Zergla]

that player's discard piles (or deck) is undefined, despite both component being defined

At the start of each player's turn, that player discards their hand, then that player draw 3 cards, then shuffle 3 random card from your discard pile to your deck.
work, but
At the start of each player's turn, that player discards their hand, then that player draw 3 cards, then shuffle 3 random card from that player's discard pile to that player's deck.
doesn't