The machine has a disclaimer saying the max payout is $2500. I’d say a message telling you you’ve won over a million is a pretty good indicator of a malfunction.
That being said, I guess the best move is to gamble in the United States, not on an Indian Reservation.
I was thinking that a $1.6 million payout on nickel slots seems extremely unlikely - they just don’t have jackpots that big on nickel slots.
This is another factor. The only slots with million-dollar payouts are "progressives", in which a large number of machines feed a common jackpot pool. If this machine is not a progressive, there is no way it could pay a million.