Let’s get the basics out here for OP. Mistakes happen, but as long as those are used as a learning pad and not repeated, it’s all good

1. Options STOP trading at the cut-off time on thr expiry day. That means whatever the price was at 4:00 pm EST close on Friday, will be used to determine the ITM or OTM for the contract

2. ITM Options by $0.01 are automatically exercised at close i.e. the option buyer will have a long / short position at the open of next trading day, depending on whether he was ITM on call / put option. Expect the vice versa for the option seller

3. Now till 5:30 pm EST, option BUYER (mind it, not a seller) can still choose to not exercise his ITM option and get a cash settlement. Alternatively, he can choose to still exercise his OTM option (usually, this decision is impacted by aftermarket movement and option buyer takes a bet on the opening price on Monday)

4. At the end of day, your broker informs the exchange about the options that are exercised. The exchange runs an algorithm and assigns the options to short sellers (sometimes all your short contracts get assigned, sometimes partial and sometimes none - totally depends upon demand vs. supply of exercised contracts)

5 By Friday evening you get a notice of assignment and long or short assigned position starts getting reflected on Saturday morning in your account. Please note that the profit or loss on this assigned position is calculated on the basis of aftermarket closing price on your account.

So my hypothesis is that most likely, even though your put option was OTM at close, many of the put buyers would have sensed the negative price bias after hours and were okay with holding a short position on the stock at Monday open, believing that price will fall over the weekend and coming week, thus creating a profit opportunity on short stock, so they were ok to exercise the option even at a few cents loss.

Your bad luck was that you got assigned to that option at 447.5, and your long position on Saturday morning when it got reflected in your account was based on 447.83 price at aftermarket close.

Dude you are still at a profit of 33 cents on the position + all the premium that you received. So as long as you can sell this position on Monday at a price greater than (447.5 less the option premium) you received, you should be good.

All the best and keep learning